Chocolate Mousse Cake Recipe

This chocolate mousse cake is three layers in one 9 inch springform pan: a thin buttermilk cocoa cake, a thick layer of chocolate chip mousse, and a glossy ganache top. It serves 12 and takes about 5 hours from start to first slice. Only about an hour of that is hands-on. The fridge does the rest.

One step decides the texture. The melted chocolate for the mousse has to cool until it feels barely warm before it meets the whipped cream. Too hot and it melts the cream; too cold and it sets into hard little flecks.

Three Layers, One Springform Pan

• Pan: 9 inch springform, greased, bottom lined with parchment
• Cake base: 1 cup flour, ⅓ cup cocoa, buttermilk, oil, one egg and hot coffee, baked at 350°F (175°C) for 25 to 30 minutes
• Mousse: 2 cups cold heavy cream whipped with 2 tablespoons powdered sugar, folded with 1½ cups semi-sweet chips melted into ¼ cup milk
• Ganache: 1 cup semi-sweet chips and ½ cup heavy cream, plus 1 tablespoon butter for shine if you like
• Chilling: at least 2 hours after the mousse goes on, then at least 4 hours or overnight after the ganache
• Serves: 12 slices, and it keeps 3 days in the fridge

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A Thin Cake Layer Made With Coffee and Buttermilk

The base is a single, fairly thin layer. Its job is to carry the mousse and give each forkful something soft to cut through, so do not expect a tall cake.

Sift the flour, cocoa, baking powder, baking soda and salt into a large bowl, then whisk in both sugars. In a second bowl, mix the buttermilk, oil, egg and vanilla. Stir the wet ingredients into the dry a little at a time, which keeps lumps away, then pour in the hot coffee last. The batter turns thin, close to heavy cream. That is how it should look.

You will not taste coffee in the finished cake. It just makes the cocoa taste darker. Hot water works if you prefer, and hot espresso pushes the chocolate flavor further. Bake for 25 to 30 minutes, until a toothpick in the center comes out clean, and start checking at 25 so the thin layer does not dry out.

Then let it cool completely, at least an hour. Even a slightly warm cake will soften the mousse from underneath.

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Folding Chocolate Into Whipped Cream Without Flecks

Melt the chocolate chips with the milk in short microwave bursts or over a double boiler, stirring until smooth. Stir in the vanilla and let it cool. You want it at about body temperature: a drop on your lip feels neither warm nor cool, and the chocolate is still completely fluid.

Whip the cold cream and powdered sugar to stiff peaks, where the tip stands straight up when you lift the whisk. Stop right there. A few seconds more and the cream goes grainy and starts heading toward butter.

Fold the chocolate into the cream with a spatula, cutting down through the middle and turning the bowl, until no white streaks remain. If you see tiny hard specks as you fold, the chocolate cooled too far. They taste fine, but next time stir a large spoonful of the cream into the chocolate first to loosen it. Chill the mousse for 15 to 20 minutes, until it holds soft mounds.

For mousse on its own, in glasses, my chocolate mousse recipe is the spoon version.

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Building the Layers in the Springform

Lift the cooled cake out of the pan, wash and re-grease the pan, and set the cake back in. A strip of parchment or acetate around the inside of the ring gives you straight, clean sides when you unmold it later.

Spoon the mousse onto the cake and spread it from the center outward with an offset spatula until the top is level. Chill for at least 2 hours, until the surface feels set and barely marks when you touch it. When I have been short on time, 20 minutes in the freezer has firmed it up enough to move on.

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Letting the Ganache Cool Before It Touches the Mousse

Heat the cream in a small saucepan until it just starts to simmer, pour it over the chocolate chips, and let it sit for 2 to 3 minutes. Stir until smooth and glossy, then add the butter if you are using it. With a little more chocolate than cream, it sets into a soft, sliceable layer rather than a hard shell.

Now wait. Hot ganache poured onto mousse melts the top into a muddy line. Give it 10 to 15 minutes, until it feels just warm on the back of your finger and flows thickly off a spoon. Pour it into the center of the chilled mousse and nudge it out to the edges with a spatula. If you want to go deeper on texture and ratios, my chocolate ganache recipe covers pouring, spreading and fixing it.

Chill the finished cake for at least 4 hours, or overnight.

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Chocolate Mousse Frosting for Layer Cakes

The mousse layer also works as a chocolate mousse frosting for cakes, as long as the cake stays cold. One batch makes roughly 4 to 5 cups, about enough to fill and frost a chilled two layer 8 inch cake. Spread a thin coat over the outside first, chill it for 20 minutes, then add the rest. It gives soft, fluffy swirls rather than sharp edges.

It is softer than buttercream, so keep the cake in the fridge until serving and skip it for a cake that will sit in a warm room for hours. For a slightly stiffer mousse made to pipe, see my chocolate mousse filling for cakes. For a frosting that stands up better at room temperature, use whipped chocolate frosting or chocolate ganache frosting.

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Grainy Mousse, Soft Slices and Other Fixes

• Hard chocolate flecks in the mousse: the chocolate was too cool when folded. They will not melt back in, so fold while it is still fluid next time.
• Grainy, curdled cream: it was whipped past stiff peaks. Stir in 2 to 3 tablespoons of cold heavy cream by hand to smooth it before folding.
• Mousse too soft to slice: the chocolate was too warm and deflated the cream, or the cake needs more time. Leave it in the fridge overnight before cutting.
• Muddy line between mousse and ganache: the ganache went on too hot. Let it cool to just warm next time.
• Dry cake base: it baked too long for such a thin layer. Pull it when the toothpick comes out clean, not when the edges pull away.

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Making It Ahead and Cutting Clean Slices

This cake is better made the day before. The overnight chill firms every layer and the slices hold their shape. Keep it covered in the fridge for up to 3 days.

To unmold, run a thin knife around the inside of the ring, then open the clasp and lift the ring away. Dip a sharp knife in hot water and wipe it dry before every cut. A tiny pinch of flaky sea salt on top of each slice plays nicely against the sweet mousse.

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Chocolate Mousse Cake Questions

Does this chocolate mousse cake need gelatin?

No. The melted chocolate in the mousse firms up as it chills, and the long chill sets it enough to slice cleanly.

Can I use chocolate bars instead of chips?

Yes. Chop the same weight, 260 g for the mousse and 170 g for the ganache. Bars melt a little more smoothly.

Can I leave out the coffee?

Yes. The recipe already allows hot water instead, and the cake bakes the same way.

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More Mousse Desserts From My Kitchen

For a taller showpiece with three mousse layers, try my triple chocolate mousse cake. If you want mousse in bar form, the chocolate mousse brownies put it on a chewy brownie, and the chocolate mousse cheesecake swirls it over a creamy filling. The rest of my chocolate desserts live in one place.