This chocolate mousse cake ticks a lot of boxes—big, grand, ideal for the holidays, and just enough of a challenge to ensure you feel accomplished. The one-bowl chocolate cake layer is dense and deeply fudgy; the mousse is airy and light; and the thin slip of ganache on top brings it all together. No matter what kind of year you’ve had, this dessert is the way to cap it all off.
Making the mousse alone will teach you three foundational pastry techniques, and they’re all hidden in plain sight. It starts with a crème anglaise; adding that to chopped chocolate yields a crémeux, and folding in whipped cream turns it into a mousse. Magical. The cake can be easily made from start to finish in the space of a morning, but can be split up further to fit your schedule. Bake the chocolate cake layer and chop all the chocolate on the first day; make the mousse and assemble the cake the following.
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Recipe information
Total Time
1 hour 30 minutes (plus 6 hours for chilling)
Yield
12 servings
Ingredients
Cake
1
1
⅓
1
½
1
⅓
⅓
1
Mousse
10
1
4
½
1
1
1¾
Ganache and Assembly
4
½
¼
Special Equipment
Need to make a substitution?
Preparation
Cake
Step 1
Place a rack in middle of oven; preheat to 350°. Lay a sheet of parchment paper over base of pan. Fit band on top and clamp shut to hold parchment in place; scrunch up around base. Wrap a sheet of foil around bottom and up sides of pan (this will help prevent any leaking).
Step 2
Whisk together 1 cup (125 g) all-purpose flour, 1 cup (200 g) sugar, ⅓ cup (28 g) Dutch-process cocoa powder, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and ½ tsp. baking soda in a medium bowl. Add 1 large egg, ⅓ cup buttermilk, ⅓ cup vegetable oil, 1 tsp. vanilla bean paste or vanilla extract, and ½ cup water and whisk until smooth, about 1 minute. Scrape batter into prepared pan.
Step 3
Bake cake until a tester inserted into the center comes out with a few moist crumbs attached and top springs back when gently pressed, 25–35 minutes. Transfer pan to a wire rack and let cake cool in pan 10 minutes. Remove band from base; slide cake off parchment and base and onto rack. Let cool completely; discard parchment and foil.
Do Ahead: Cake can be baked 1 day ahead. Store tightly wrapped at room temperature.
Mousse
Step 4
Place 10.5 oz. bittersweet chocolate, preferably 70% cacao, coarsely chopped, in a large heatproof bowl. Bring 1 cup whole milk to a simmer in a small saucepan over medium, about 5 minutes.
Step 5
Meanwhile, vigorously whisk 4 large egg yolks and ½ cup (100 g) sugar in a small heatproof bowl until smooth and lemon yellow, about 1 minute.
Step 6
Gradually stream hot milk into yolk mixture, whisking constantly until combined. Return to pan, scraping bowl clean with a heatproof rubber spatula. Cook over medium, stirring constantly, until thickened just enough to coat a spoon, about 5 minutes. (Do not let custard come to a boil or it will curdle.) Immediately pour over chocolate; let sit 1 minute. Add 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt and 1 tsp. vanilla bean paste or vanilla extract and whisk vigorously until chocolate is melted and crémeux is smooth.
Step 7
Beat 1¾ cups heavy cream in the bowl of a stand mixer fitted with the whisk attachment on medium speed until soft peaks form, about 5 minutes. Add one quarter of whipped cream to crémeux; mix until combined (this will lighten it). Add remaining whipped cream in 3 additions, gently folding after each until incorporated before adding more.
Step 8
Reassemble springform pan; line bottom with a parchment paper round. Wrap a strip of parchment around inside of pan to create a collar that stands about 1" above band (ends should overlap by at least 3"). Using a large serrated knife, trim off mound on top of cake to create a flat surface if needed. Return cake to pan. Spoon mousse over and smooth into an even layer with an offset spatula or spoon. Place pan on a rimmed baking sheet; chill until top of mousse feels set, about 2 hours.
Ganache and Assembly
Step 9
Place 4 oz. bittersweet chocolate, preferably 70% cacao, coarsely chopped, in a medium heatproof bowl. Bring ½ cup heavy cream and ¼ tsp. Diamond Crystal or Morton kosher salt to a bare simmer in a small saucepan over medium. Pour over chocolate and let sit 2 minutes. Whisk gently until smooth. If ganache feels hot, let cool until lukewarm but still pourable, 8–10 minutes.
Step 10
Pour ganache over mousse and smooth with offset spatula. Chill until mousse is completely set (ganache will set in about 30 minutes), at least 4 hours and up to 12 hours (no need to cover unless fridge has other strong smells).
Step 11
To serve, remove band from base and peel parchment strip away from sides of cake. Transfer cake with base to a plate and let sit 20 minutes; sprinkle top with flaky sea salt.