Chocolate Mousse Cake Recipe

Prepare and preheat

Preheat the oven to 350°F (175°C). Grease and line the bottom of an 8-inch (20 cm) springform pan with parchment paper; grease the sides lightly.

Mix dry ingredients

In a medium bowl, whisk together the flour, 3/4 cup granulated sugar, cocoa powder, baking powder, baking soda, and 1/2 tsp salt until evenly combined.

Mix wet ingredients

In a separate bowl, whisk the egg, milk, vegetable oil, and 1 tsp vanilla extract until combined.

Combine and add hot water

Pour the wet ingredients into the dry ingredients and stir until just combined. Gradually stir in 1/2 cup hot water until batter is smooth and slightly thin (this yields a moist cake).

Bake

Pour the batter into the prepared 8-inch pan and smooth the top. Bake in the preheated oven for 20–25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs but not raw batter.

Cool

Remove pan from oven and let cake cool in the pan on a wire rack for 15 minutes. Run a knife around the edge, then release the springform and carefully remove the ring. Allow the cake layer to cool completely to room temperature on the rack (about 30–45 minutes) before assembling mousse layer.

Melt chocolate (ganache base)

Chop 8 oz semisweet chocolate and place in a heatproof bowl. Heat 1/4 cup heavy cream just until steaming (do not boil) and pour it over the chocolate. Let rest 1 minute, then stir until smooth. Stir in 2 tbsp unsalted butter and 1/2 tsp instant espresso powder (if using) until fully incorporated. Set aside to cool to lukewarm.

Temper egg yolks

In a small heatproof bowl, whisk together 3 egg yolks and 1/4 cup granulated sugar until pale and slightly thickened. Place the bowl over (not in) a pot of simmering water (double-boiler) and whisk constantly for 3–4 minutes until the mixture is warm, slightly thickened and registers about 140°F–150°F if using a thermometer. Remove from heat.

Combine yolks with chocolate

Gradually whisk about 1/4 cup of the melted chocolate into the warmed yolks to temper, then whisk the tempered yolk mixture back into the remaining melted chocolate until smooth and glossy. Stir in 1/8 tsp fine salt and 1/2 tsp vanilla extract. Allow mixture to cool to room temperature but still fluid.

Whip cream

In a chilled bowl, whip 1.5 cups cold heavy cream with an electric mixer to soft peaks, then add a small spoonful (about 1–2 tsp) of the reserved granulated sugar if desired and whip to medium peaks — the mousse needs structure but should still be light.

Fold cream into chocolate

Fold one-third of the whipped cream into the chocolate base to lighten it, using a rubber spatula and gentle motions. Fold in the remaining whipped cream in two additions until the mousse is homogenous and smooth, taking care not to overmix (maintain airy texture).

Assemble mousse on cake

Place the cooled cake layer back into the springform ring on a cake board or plate. Spoon the mousse over the cake, smoothing the top with an offset spatula to an even layer about 1.25–1.5 inches thick (or to desired height). Tap the pan gently on the counter to remove any large air pockets.

Chill to set

Cover the assembled cake loosely with plastic wrap and refrigerate for at least 4 hours, preferably overnight, until the mousse is fully set and firm to the touch.

Prepare glaze

Shortly before serving, chop 3 oz chocolate and place in a small bowl. Heat 1/4 cup heavy cream until steaming and pour over the chocolate. Let sit 1 minute, then stir until smooth. Stir in 1 tbsp unsalted butter until glossy. Let the glaze cool slightly but remain pourable (about 5 minutes).

Glaze the cake

Remove cake from refrigerator and release the springform ring. Place the cake on a wire rack over a sheet pan to catch drips. Pour the glaze over the center of the mousse, using an offset spatula to help it reach the edges and drip slightly down the sides for a smooth finish. If desired, dust the top lightly with 1 tsp cocoa powder.

Garnish and serve

Transfer cake to serving plate. Arrange fresh berries and mint leaves around or on top as desired. For clean slices, chill the glazed cake for 15–20 minutes to set the glaze, then run a sharp knife under hot water, dry, and slice. Wipe the knife between cuts for neat slices.