Brown Butter Layer Cake with Dark Chocolate Mousse Filling

This gorgeous layer cake caught my eye in the February Food and Wine issue and just in time for my friend Bonnie's birthday dinner.  The instructions for making this cake are pretty time consuming but they are simple and this cake is worth the time spent to make it.

Brown Butter Layer Cake

3 sticks unsalted butter (12 ounces), plus more for greasing the pans

2 1/4 cups all-purpose flour, plus more for dusting

2 1/4 teaspoons baking powder

1 teaspoon salt

1 2/3 cups sugar

1 vanilla bean, split and seeds scraped

3 large egg yolks

2 large eggs

1 1/4 cups milk, at room temperature

Mousse filling and Buttercream frosting (see recipes below)

Preheat the oven to 325°. Butter two 9-inch cake pans and line the bottoms of the pans with parchment paper. Butter the paper and dust the pans with flour, tapping out the excess.

In a medium saucepan, melt the 3 sticks of butter. Cook over low heat, stirring occasionally, until foamy, about 5 minutes. Continue to cook, stirring frequently, until the milk solids turn brown and the butter smells nutty, about 4 minutes longer. Scrape the melted butter and browned bits into a large heatproof bowl. Set the bowl in an ice water bath until the butter begins to set around the edge, about 8 minutes.

Meanwhile, in a medium bowl, whisk the 2  1/4 cups of all-purpose flour with the baking powder and salt.

Remove the bowl from the ice water and scrape up the hardened butter. Transfer the butter to the bowl of a standing electric mixer fitted with the paddle and beat until creamy. Add the sugar and vanilla seeds and beat at medium-high speed until fluffy, about 3 minutes. Beat in the egg yolks followed by the whole eggs. Beat in the dry ingredients and milk in 3 alternating additions, scraping down the side and bottom of the bowl as necessary.

Pour the cake batter into the prepared pans and bake in the center of the oven for about 40 minutes, rotating the pans halfway through baking, until the cakes are golden and a toothpick inserted in the centers comes out clean. Cool the cakes in the pans for 20 minutes, then invert them onto a rack to let them cool completely. Peel off the parchment paper.

Set one cake layer on a platter. Spread the mousse filling (recipe below) on top and cover with the second cake layer. Frost the cake all over with a thin layer of buttercream and refrigerate until set, about 5 minutes. Frost the cake with the remaining buttercream (recipe below). Refrigerate the cake until the frosting is firm, at least 15 minutes, before serving.

Dark Chocolate Mousse Filling

1/2 teaspoon unflavored gelatin

1/2 tablespoon water

1/2 cup chilled heavy cream

2 tablespoons coffee liqueur

2 large egg yolks

2 tablespoons sugar

Pinch of salt

4 ounces bittersweet chocolate, melted and cooled

In a small bowl, sprinkle the gelatin over the water and let stand until softened. Meanwhile, in a medium bowl, using a handheld electric mixer, beat the cream until softly whipped. Refrigerate until chilled, about 10 minutes.

In a small, microwave-safe bowl, heat the coffee liqueur at high power until hot, about 45 seconds. Stir in the softened gelatin until dissolved.

In another medium bowl, beat the egg yolks with the sugar and salt at high speed until pale and thickened, about 5 minutes. While beating the yolks, beat in the coffee-gelatin liquid; scrape the side and bottom of the bowl. Beat in the melted chocolate. Using a rubber spatula, fold in the whipped cream in 2 additions. Scrape the mousse into a bowl, cover with plastic wrap and refrigerate until firm, at least 4 hours or overnight.

Rich Chocolate Buttercream

1 cup sugar

3 large egg whites

1/4 teaspoon salt

1/2 teaspoon pure vanilla extract

2 sticks plus 2 tablespoons unsalted butter (9 ounces), cut into tablespoons, at room temperature

4 ounces extra-bittersweet chocolate, melted and cooled

In a blender or food processor, pulse the sugar until powdery. Transfer the sugar to a medium heatproof bowl and whisk in the egg whites and salt. Set the bowl over a saucepan of simmering water and whisk gently until the sugar is completely dissolved, 5 minutes.

Transfer the warm egg-white mixture to the bowl of a standing electric mixer fitted with the whisk. Add the vanilla and beat at medium-high speed until stiff and glossy, about 8 minutes. Beat in the butter a few pieces at a time, making sure it is fully incorporated before adding more. The buttercream should be light and fluffy; if it appears runny at any time, transfer the bowl to the freezer for 5 to 10 minutes, then return it to the mixer and continue.

Beat in the melted chocolate until fully incorporated, scraping down the side and bottom of the bowl. Remove the bowl from the mixer and beat with a wooden spoon to remove any air bubbles.