This chocolate peanut butter cake is a moist chocolate sheet cake baked in one 9×13 inch pan, topped with an optional layer of chocolate fudge and a thick spread of peanut butter frosting. It bakes for 35 to 40 minutes at 350°F, needs about an hour to cool, and cuts into 16 squares.
Boiling water and sour cream are what keep the crumb soft for days. I have made it for birthdays, office parties, and plain weekends, and it travels in its pan with the frosting on. If you just want the frosting, my peanut butter frosting has its own page.
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Try the Recipe Converter →One 9×13 Pan, 16 Squares: The Details
- Pan: one greased 9×13 inch baking pan
- Oven: 350°F (175°C) for 35 to 40 minutes
- Time: 25 minutes prep, about 1 hour cooling, about 2 hours 5 minutes in total
- Yield: 16 servings
- Cake: cocoa, oil, whole milk, 2 eggs, 1 cup boiling water, ½ cup sour cream
- Fudge layer (optional): 1 cup semi-sweet chocolate chips melted with cream, butter and vanilla
- Frosting: 1 cup creamy peanut butter (not natural), butter, 2½ to 3 cups powdered sugar
Why Boiling Water and Sour Cream Keep It Moist
Hot water dissolves the cocoa completely and brings out a darker, deeper chocolate flavor than cold liquid would. It also makes a very thin batter, and thin batter bakes into a tender, damp crumb. Oil instead of butter keeps the cake soft even straight from the fridge.
Sour cream goes in last. It adds a little tang that cuts through the sweet frosting and keeps the squares moist for several days. Use unsweetened cocoa powder only. Sweetened cocoa or drinking chocolate throws off the sugar and the flavor.
Mixing the Chocolate Batter in Two Bowls
Sift the flour, cocoa, baking powder, baking soda, and salt into one large bowl. Cocoa clumps easily, and sifting also spreads the two leaveners evenly so the cake rises flat.
In a second bowl, whisk the sugar with the eggs, milk, oil, and vanilla until smooth. The sugar goes with the wet ingredients here, not the dry ones, so it starts dissolving right away. Room temperature eggs and milk blend more smoothly and give a more even crumb.
Stir the dry ingredients into the wet ones just until combined. Overmixing builds gluten and makes the cake tight and dense.
Now pour in the boiling water slowly, stirring as you go. The batter will look alarmingly thin. That is right. Fold in the sour cream last, until the batter is smooth and slightly glossy.
Knowing When the Sheet Cake Is Done
Pour the batter into the greased pan and bake for 35 to 40 minutes. The cake is ready when the center springs back when pressed lightly, the edges just begin to pull away from the pan, and a toothpick in the middle comes out clean or with a few moist crumbs.
Do not wait for a bone dry toothpick. In a 9×13 the edges overbake long before the center looks dry. Cool the cake completely in the pan on a wire rack. Any warmth left in it melts the fudge and slides the frosting.
The Optional Fudge Layer
Melt the chocolate chips, heavy cream, and butter in a saucepan over medium heat, stirring constantly until smooth. Take it off the heat, stir in the vanilla, and let it cool for a few minutes so it is warm but not hot.
Spread it over the cooled cake and let it set before frosting. It is ready when the surface turns matte and no longer sticks to a fingertip, about 30 minutes in the fridge. Frosting over wet fudge smears the two together. I add this layer whenever the cake will sit out at a party for a few hours; it stays creamy and gives every square a ribbon of chocolate. For a thicker filling in other bakes, try this chocolate cake filling.
Peanut Butter Frosting That Spreads Smoothly
Beat the peanut butter and butter until smooth and fluffy. Add the powdered sugar gradually, alternating with the heavy cream, then the vanilla and a pinch of salt. The salt matters. It keeps the frosting from tasting flat next to the chocolate.
Use a commercial, no-stir creamy peanut butter. Natural peanut butter separates and turns the frosting greasy. Brands vary in thickness, which is why the recipe gives 2½ to 3 cups of sugar: too stiff, add a tablespoon of cream; too loose, add ¼ cup more sugar. You want it to hold soft peaks and spread without tearing the cake.
Spread the frosting over the cake (or the set fudge) right in the pan and make soft waves with a spatula. For a thinner, glossy topping you can pour, use peanut butter icing instead.
Serving It as a Peanut Butter Birthday Cake
Because it is frosted in the pan, this cake is easy to carry to a party. Cover the pan, keep it level, and cut it on site. A 4 by 4 grid gives 16 even squares; wipe the knife between cuts so the fudge and frosting stay neat. Candles stand up well in the thick frosting.
Chocolate Peanut Butter Sheet Cake vs Layer Cake
The sheet cake is simpler: one pan, no stacking, and the softest texture. For a tall birthday cake, the same batter fills two 9 inch round pans. Start checking them at about 28 minutes, since shallower layers bake faster. Spread the fudge between the layers once it has set slightly, and make half a batch more frosting to cover the sides.
Sour Cream or Buttermilk?
In my notes, buttermilk made a slightly airier cake but less rich, and it dried out sooner. Sour cream keeps more moisture and gives a sturdier crumb, so I always come back to it. Full fat plain Greek yogurt is the closest swap.
Fixing a Sunken, Dry or Smeared Cake
- Sunken middle: it came out too early or the oven door was opened in the first 25 minutes. Check the center with a toothpick before pulling it.
- Dry edges: it baked too long. Start checking at 35 minutes and trust a few moist crumbs.
- Fudge runs or smears: the cake was still warm or the fudge had not set. Chill until matte before frosting.
- Greasy or grainy frosting: natural peanut butter or butter that was too soft. Chill the bowl for 10 minutes and beat again.
Keeping Leftover Squares Soft
Store the cake covered at room temperature for up to 2 days, or in the fridge for up to 5 days. Let refrigerated squares sit out for 30 minutes before serving so the frosting softens and the cake tastes moist again. You can bake and frost it a day before a party.
More Chocolate and Peanut Butter Bakes
For a lighter cake with no chocolate base, try my Reese’s peanut butter cake or this peanut butter sheet cake. A plain sour cream chocolate cake uses a similar moist crumb. For smaller treats, bake chocolate peanut butter brownies, buckeyes, or easy peanut butter cookies, and for a no-oven dessert, a chocolate peanut butter pie.
Peanut Butter Cake Questions
Can I use crunchy peanut butter in the frosting?
Yes, as long as it is a no-stir commercial brand. The frosting will have small peanut pieces and will not spread quite as smoothly.
Can I skip the fudge layer?
Yes. Spread the peanut butter frosting straight onto the fully cooled cake; it is still a complete chocolate cake with peanut butter frosting.
Chocolate Peanut Butter Cake Recipe
This chocolate peanut butter cake is a moist chocolate sheet cake made with cocoa, oil, whole milk, sour cream, and boiling water, topped with an optional fudge layer and a fluffy peanut butter frosting. It bakes in one 9x13 inch pan in 35 to 40 minutes, takes about 2 hours with cooling, and cuts into 16 squares. The boiling water blooms the cocoa and makes a thin batter that bakes into a soft crumb that stays moist for days. Serve it as a peanut butter birthday cake, a party sheet cake you can carry in its pan, or bake the batter in two 9 inch rounds for a chocolate cake with peanut butter frosting. It keeps 2 days covered or 5 days in the fridge.
Ingredients
- FOR THE CHOCOLATE CAKE
- 1 ¾ cups all-purpose flour
- ¾ cup unsweetened cocoa powder
- 1 ½ teaspoons baking powder
- 1 ½ teaspoons baking soda
- 1 teaspoon salt
- 2 cups granulated sugar
- 2 large eggs
- 1 cup whole milk
- ½ cup vegetable oil
- 2 teaspoons vanilla extract
- 1 cup boiling water
- ½ cup sour cream
- FOR THE FUDGE FILLING (optional but highly recommended)
- 1 cup semi-sweet chocolate chips
- ½ cup heavy cream
- 2 tablespoons butter
- 1 teaspoon vanilla extract
- FOR THE PEANUT BUTTER FROSTING
- 1 cup creamy peanut butter (not natural)
- ½ cup unsalted butter, room temperature
- 2 ½ to 3 cups powdered sugar
- ¼ cup heavy cream (add more for a lighter texture)
- 1 teaspoon vanilla extract
- Pinch of salt
Instructions
- PREHEAT AND PREP: Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking pan and set aside.
- MIX DRY INGREDIENTS: In a large bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt to combine evenly.
- WHISK WET INGREDIENTS: In a separate large bowl, whisk the sugar, eggs, milk, vegetable oil, and vanilla extract until smooth and fully blended.
- COMBINE BATTER: Add the dry ingredients to the wet mixture and stir gently until just combined without overmixing.
- ADD BOILING WATER: Gradually stir in the boiling water to thin the batter. The mixture will be loose—this creates a moist cake texture.
- ADD SOUR CREAM: Fold in the sour cream until completely incorporated. The batter should be smooth and slightly glossy.
- BAKE THE CAKE: Pour the batter into the prepared pan and bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean.
- COOL COMPLETELY: Remove the cake from the oven and let it cool fully in the pan on a wire rack before adding filling or frosting.
- MAKE THE FUDGE FILLING (OPTIONAL): In a medium saucepan over medium heat, melt the chocolate chips, heavy cream, and butter, stirring constantly until smooth. Remove from heat, stir in vanilla, and let cool slightly. Spread evenly over the cooled cake and let it set before frosting.
- MAKE THE PEANUT BUTTER FROSTING: In a large mixing bowl, beat the peanut butter and butter together until smooth and fluffy. Add powdered sugar gradually, alternating with the heavy cream, and beat until creamy. Mix in the vanilla and a pinch of salt. Adjust consistency with additional cream or sugar as needed for easy spreading.
- FROST THE CAKE: Once the cake and optional filling have cooled and set, spread the peanut butter frosting generously over the top. Use a spatula to create soft waves for a clean, decorative finish.
Notes
Use a commercial creamy peanut butter; natural peanut butter separates and makes the frosting greasy. Brands vary, so adjust the frosting with a tablespoon of cream or ¼ cup more powdered sugar. Pull the cake when the center springs back and a toothpick shows a few moist crumbs; overbaking dries the edges. Cool the cake completely before adding the fudge. The fudge layer is optional; let it set until matte, about 30 minutes in the fridge, before frosting. Store covered at room temperature for up to 2 days or in the fridge for up to 5 days, and let it sit out 30 minutes before serving.
