This moist chocolate sheet cake is a one bowl buttermilk chocolate cake baked in a 9×13 inch pan and covered with a warm chocolate fudge frosting. It takes about 50 minutes, including 32 to 36 minutes in a 350°F oven, and cuts into 12 big squares or up to 20 smaller ones for a crowd.
The trick is a cup of hot water or hot coffee stirred into the batter at the end. The batter looks far too thin, and that is exactly what makes the crumb soft and moist. The frosting is made in a saucepan and spread over the cake while it is still slightly warm, so it settles into a smooth, glossy layer.
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- Pan: one 9×13 inch pan, greased or lined with parchment
- Serves: 12 large squares, or up to 20 smaller pieces
- Time: 15 minutes prep, 32 to 36 minutes baking, 20 minutes cooling
- Oven: 350°F (175°C)
- Moisture secret: 1 cup hot water or coffee, plus buttermilk and oil
- Keeps: 3 days covered at room temperature, longer in the fridge, freezes in squares
Why the Batter Is So Thin
The cake starts like most chocolate cakes: whisk the flour, sugar, cocoa, baking soda, baking powder, and salt so the leaveners spread evenly, then beat in the buttermilk, oil, eggs, and vanilla. The last step is the one that matters. Pour in the hot liquid slowly on low speed.
Hot liquid blooms the cocoa, which brings out more chocolate flavor, and loosens the batter until it is almost pourable. Thin batter bakes into a soft, sponge like crumb that stays moist for days. If you use hot coffee, the cake will not taste like coffee; it just makes the chocolate taste deeper. I use coffee every time, even if it means sneaking a cup from the pot before anyone else wakes up.
Ingredients That Keep It Moist
Buttermilk adds tang and tenderness, and vegetable oil keeps the cake soft even straight from the fridge, which butter based cakes cannot do. Two room temperature eggs bind everything, and vanilla rounds out the cocoa. Use room temperature buttermilk and eggs so the batter mixes smoothly in 1 to 2 minutes.
There is no stand mixer needed, no layers to level, and no fragile cake to move. Just one bowl, one pan, and a whisk or hand mixer.
Baking and Cooling
Pour the batter into the prepared pan and tap it on the counter to release air bubbles. Bake for 32 to 36 minutes, until a toothpick in the center comes out clean. Start checking at 32 minutes; an overbaked sheet cake dries out at the edges first.
Let the cake cool in the pan on a rack for at least 20 minutes. You want it slightly warm, not hot, when the frosting goes on.
Chocolate Sheet Cake With Chocolate Frosting
The chocolate frosting is a warm fudge frosting made in a saucepan. Melt the butter over low heat, stir in the cocoa until smooth, and take it off the heat. Then whisk in the powdered sugar and milk alternately until thick, smooth, and creamy, and finish with vanilla and a pinch of salt. Add a little more milk if needed; it should be thick but spreadable.
Spread the warm frosting over the slightly warm cake. It spreads itself, settles into the top, and sets into a soft layer that keeps its shine. Swirl it with the back of a spoon if you like. Sometimes I pour it straight from the pan and call it done. If you prefer a whipped, no cook topping to pile on cooled cake, my chocolate fudge frosting is a good match.
How It Compares to Texas Sheet Cake
People often ask whether this is a Texas sheet cake. It is a close cousin. A Texas sheet cake is usually thinner, baked in a larger, shallower pan, and its batter is made by boiling butter, water, and cocoa together. This version uses buttermilk, oil, and hot coffee in a deeper 9×13 inch pan, so each square is thicker and more cake like, with a softer crumb.
Both share the same idea: warm frosting on a warm cake. If you want the vanilla version of an easy party cake, try my vanilla sheet cake.
What Can Go Wrong
- Dry cake: overbaked. Start checking at 32 minutes and pull it as soon as a toothpick comes out clean.
- Sunken center: underbaked, or the oven door was opened early.
- Frosting too stiff to spread: it cooled in the pan. Rewarm it gently with a splash of milk.
- Frosting melted and ran off: the cake was still hot. Wait the full 20 minutes.
- Bitter edge: too strong a coffee. Use regular brewed coffee or hot water.
Serving Ideas
Cut it into 12 big squares, or stretch it to 20 for a crowd. Sprinkle flaky sea salt over the frosting for contrast, or press sprinkles into it while it is still soft for a birthday. My daughter once added mini marshmallows on top, and that batch disappeared in ten minutes at a block party.
Serve it warm with a scoop of ice cream or cold with coffee. For a party table with more choices, set it out with a brownie cake or squares of simple birthday cake.
Storing and Freezing Slices
Keep the cake covered at room temperature for up to 3 days, in a dish with a lid or wrapped tightly in foil. For longer, refrigerate it. The oil based crumb stays soft, but let cold slices sit for 15 minutes so the crumb and frosting relax.
To freeze, wrap individual squares in plastic and foil. They thaw quickly and make great emergency desserts.
More Chocolate Cakes
If this becomes a favorite, try the Guinness chocolate cake, a chocolate olive oil cake, or a sour cream chocolate cake. For a frosting with coconut and pecans, there is German chocolate frosting, and for something lighter, a white chocolate cake.
Chocolate Sheet Cake FAQ
What size pan do I need for chocolate sheet cake?
A 9×13 inch pan. The cake bakes at 350°F for 32 to 36 minutes.
Why is my chocolate sheet cake batter so thin?
The cup of hot water or coffee makes it thin on purpose. That thin batter is what bakes into a moist, soft crumb.
Should I frost a sheet cake warm or cold?
Slightly warm. Let it cool 20 minutes, then spread the warm frosting so it settles into a smooth, glossy layer.
Can I use water instead of coffee?
Yes. Hot water works fine. Coffee just deepens the chocolate flavor without tasting like coffee.
How many people does a 9×13 chocolate sheet cake serve?
12 generous squares, or up to 20 smaller pieces for a party.
Save this chocolate sheet cake to your easy crowd pleasers board. If you make it, tell me in the comments whether you went with coffee or water and which toppings you added.
Ultra-Moist Chocolate Sheet Cake with Fudge Frosting
This chocolate sheet cake is a moist, one bowl buttermilk chocolate cake baked in a 9x13 inch pan and topped with a warm chocolate fudge frosting. It takes about 50 minutes, baking at 350°F for 32 to 36 minutes, and serves 12 large squares or up to 20 smaller pieces. A cup of hot water or hot coffee is stirred into the batter last, which makes it thin and bakes into a soft, tender crumb, while oil keeps the cake moist for days. The chocolate frosting is made in a saucepan and spread over the warm cake so it sets glossy. Serve this chocolate sheet cake with chocolate frosting for birthdays, potlucks, and parties. It keeps 3 days covered and freezes well in squares.
Ingredients
- FOR THE CAKE
- 2 cups all-purpose flour
- 2 cups granulated sugar
- ¾ cup unsweetened cocoa powder
- 2 tsp baking soda
- 1 tsp baking powder
- ½ tsp salt
- 1 cup buttermilk (room temperature)
- ½ cup vegetable oil
- 2 large eggs (room temperature)
- 1 tsp pure vanilla extract
- 1 cup hot water or hot coffee
- FOR THE FUDGE FROSTING
- ½ cup (1 stick) unsalted butter
- ½ cup unsweetened cocoa powder
- 5 cups powdered sugar
- 1/3 cup whole milk (plus more if needed)
- 1 tsp pure vanilla extract
- Pinch of salt
Instructions
- PREPARE THE PAN AND OVEN: Preheat the oven to 350°F (175°C). Grease a 9x13-inch pan or line it with parchment paper to prevent sticking.
- MIX THE DRY INGREDIENTS: In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt until evenly combined and no lumps remain.
- ADD THE WET INGREDIENTS: Add the buttermilk, vegetable oil, eggs, and vanilla extract to the dry ingredients. Beat with a mixer or whisk by hand until the batter is smooth and well incorporated, about 1–2 minutes.
- INCORPORATE THE HOT LIQUID: Slowly pour in the hot water or hot coffee while mixing on low speed. The batter will be thin—this is expected and helps create a moist texture.
- BAKE THE CAKE: Pour the batter into the prepared pan and gently tap the pan on the counter to release any air bubbles. Bake for 32–36 minutes or until a toothpick inserted into the center comes out clean.
- COOL THE CAKE: Let the cake cool in the pan on a wire rack for at least 20 minutes before applying the frosting. The cake should be slightly warm but not hot.
- MAKE THE FROSTING BASE: In a medium saucepan, melt the butter over low heat. Once melted, stir in the cocoa powder until the mixture is smooth and combined, then remove from heat.
- WHISK IN REMAINING INGREDIENTS: Alternately add powdered sugar and milk to the saucepan, whisking continuously until the mixture is smooth and creamy. Stir in the vanilla extract and a pinch of salt. Adjust the consistency by adding a bit more milk if needed—it should be thick but spreadable.
- FROST THE CAKE: Spread the warm frosting evenly over the slightly cooled cake. Use the back of a spoon or an offset spatula to create gentle swirls across the surface.
Notes
For a deeper chocolate flavor, use hot coffee instead of water; the cake will not taste like coffee. The batter will be thin, which is expected. Let the cake cool for about 20 minutes so it is slightly warm before frosting; the frosting sets best on a warm cake. Store the cake covered at room temperature for up to 3 days, or refrigerate for longer freshness and let slices sit for 15 minutes before serving. Freeze individual squares wrapped in plastic and foil.
Nutrition Information
Yield
12Serving Size
1Amount Per Serving Calories 668Total Fat 19gSaturated Fat 6gTrans Fat 0gUnsaturated Fat 11gCholesterol 47mgSodium 465mgCarbohydrates 118gFiber 3gSugar 94gProtein 7g
