Chocolate Mousse Cake (Layered Cake with Silky Mousse)

This chocolate mousse cake pairs a moist, easy chocolate sponge with a light, silky chocolate mousse that fills the middle and covers the outside. The mousse is made with chocolate, cream, a little powdered sugar and vanilla — no eggs and no gelatine — so it is simple and reliable. The cake needs a few hours in the fridge to set, which makes it a perfect make-ahead dessert for birthdays and dinner parties. For a coffee twist, try our mocha mousse cake.

Why you'll love this

• Two textures. Moist cake and an airy, melt-in-your-mouth mousse.
• Egg-free mousse. Just chocolate, cream, sugar and vanilla.
• Make-ahead friendly. It needs to chill, so it is ready when guests arrive.
• Impressive but easy. No special equipment beyond a mixer.
• Rich but light. The mousse keeps every slice from feeling heavy.
• Customisable. Top with berries, shavings or cocoa.

Key ingredients & how to use them

• Buttermilk chocolate cake — a simple one-bowl sponge with cocoa, oil and buttermilk that stays moist under the mousse.
• Unsweetened cocoa powder — half a cup for the cake. Sift it with the flour to remove lumps.
• Buttermilk — one cup. Or stir a tablespoon of vinegar or lemon juice into a cup of milk and let it stand for 5 minutes.
• Semi-sweet or bittersweet chocolate — 225 g, finely chopped, for the mousse. Use a good bar; it is the main flavour.
• Heavy cream — two cups, divided: one cup heated to melt the chocolate, one cup whipped to lighten it.
• Powdered sugar and vanilla — to sweeten and stabilise the whipped cream.

How to make it (step by step)

1. 1

   Prepare the pan

   Heat the oven to 350°F (175°C). Grease a 9-inch round cake pan and line the base with parchment.

2. 2

   Mix the dry ingredients

   Sift the flour, cocoa powder, sugar, baking powder, baking soda and salt into a large bowl and whisk to combine.

3. 3

   Mix the wet ingredients

   In another bowl, beat the eggs, oil, buttermilk and vanilla until smooth.

4. 4

   Make and bake the batter

   Add the wet ingredients to the dry and mix just until combined. Pour into the pan, smooth the top and bake for 25 to 30 minutes, until a toothpick in the centre comes out clean. Cool in the pan for 10 minutes, then on a rack until completely cold, about 1 hour.

5. 5

   Melt the chocolate

   Put the chopped chocolate in a heatproof bowl. Heat 1 cup of the cream until it just begins to simmer, pour it over the chocolate, leave for 2 minutes, then whisk gently until smooth. Let it cool to room temperature.

6. 6

   Whip and fold the mousse

   Whip the remaining 1 cup of cream with the powdered sugar and vanilla to stiff peaks. Fold a third into the cooled chocolate to loosen it, then gently fold in the rest until smooth and airy. The chocolate must be cool, or it will melt the cream.

7. 7

   Assemble the cake

   Slice the cooled cake horizontally into two even layers with a serrated knife. Place one layer on a plate, spread with a thick layer of mousse, add the second layer and cover the top and sides with the remaining mousse.

8. 8

   Chill and serve

   Refrigerate for at least 4 hours, or overnight, until the mousse is set. Dust with cocoa or top with chocolate shavings, and serve with whipped cream or fresh berries.

Troubleshooting

• The mousse is runny. The chocolate was still warm when the cream went in, or the cream was under-whipped. Cool the chocolate and whip to stiff peaks.
• The mousse is grainy. The chocolate seized or the cream was over-whipped. Fold gently and stop whipping at stiff peaks.
• The cake domed. Level it with a serrated knife before splitting.
• The layers slid. The cake was still warm. Cool it completely.
• Messy slices. Chill the cake fully and cut with a hot, dry knife, wiping between slices.
• Dry cake. It was overbaked. Start checking at 25 minutes.

How the two-ingredient mousse works

Classic chocolate mousse is often made with whipped egg whites and yolks. This version is simpler: hot cream melts the chocolate into a smooth ganache, and whipped cream folded in afterwards supplies the air. As the cake chills, the cocoa butter in the chocolate firms up and holds those air bubbles in place, which is why the mousse sets enough to slice without gelatine.

The one rule is temperature. If the ganache is warm, it melts the whipped cream and the mousse collapses into a sauce. Let it cool until it feels neutral against your lip, but is still fluid, before folding. If it has set too firm, warm it for a few seconds and stir until it loosens again.

Serving ideas

Serve chilled slices with fresh raspberries or strawberries and a little whipped cream. For a chocolate lover’s dessert table, add our no-bake chocolate cheesecake and triple chocolate cake.