• Make-Ahead

Ingredients

CAKE

1. Fine dry bread crumbs, for dusting
2. 2 cups walnut halves (about 7 ounces)
3. 1/4 cup unsweetened cocoa powder (not Dutch process)
4. 1/2 cup cake flour
5. 1 teaspoon instant espresso powder
6. 8 large eggs, separated
7. 1 cup sugar
8. 2 teaspoons pure vanilla extract
9. 1/4 teaspoon salt

FILLING

1. 6 large egg yolks
2. 1/2 cup sugar
3. Pinch of salt
4. 2 tablespoons instant espresso powder dissolved in 1/2 cup hot water
5. 1 pound unsalted butter, softened
6. 3 tablespoons coffee liqueur
7. 1 tablespoon pure vanilla extract
8. 1/2 cup dark rum, for brushing
9. 1 1/2 cups finely chopped walnuts (about 5 ounces), plus walnut halves for garnish

Directions

1. MAKE THE CAKE: Preheat the oven to 350°. Butter two 9-inch round cake pans and line the bottoms with parchment paper. Butter the parchment and dust with bread crumbs, tapping out any excess. In a food processor, pulse the walnuts until finely chopped. Add the cocoa powder, cake flour and espresso powder and pulse until the walnuts are finely ground. Transfer to a bowl.
2. In a large bowl, using an electric mixer, beat the yolks at medium speed until thick and pale, 3 minutes. Beat in 2/3 cup of the sugar; increase the speed to high and beat until thick, about 5 minutes. Beat in the vanilla. In a medium bowl, using clean beaters, beat the egg whites and salt at medium speed until soft peaks form. Gradually add the remaining 1/3 cup of sugar and beat at high speed until the whites are stiff and glossy. Fold the egg whites and ground walnuts into the yolk mixture in 3 batches; fold just until no streaks remain.
3. Divide the batter between the prepared pans; smooth the tops. Bake for about 20 minutes, until the cakes are golden and springy. Transfer to a rack for 10 minutes, then invert the cakes and cool completely. Peel off the parchment; turn the cakes right side up.
4. MAKE THE FILLING: Fill a medium bowl with ice water. In a medium heatproof bowl, whisk the yolks, sugar and salt. Gradually whisk in the dissolved espresso. Place the bowl over a saucepan of barely simmering water and cook, stirring with a spatula, until the yolks are thick enough to coat the spatula and an instant-read thermometer inserted into the mixture registers 175°, about 10 minutes. Set the bowl in the ice bath and cool, stirring occasionally.
5. In a large bowl, using an electric mixer, beat the butter at medium speed until smooth. Beat in the yolk mixture in 4 batches, beating well between additions. Beat in the liqueur and vanilla.
6. Using a serrated knife, split each cake in half horizontally. Set a top layer, cut side up, on a cake plate; brush it with 2 tablespoons of the rum and spread with 1/2 cup of the filling. Top with a bottom cake layer; brush with 2 more tablespoons of rum and spread with another 1/2 cup of filling. Repeat with the remaining top cake layer, ending with the bottom layer, cut side down. Brush the top of the cake with the remaining 2 tablespoons of rum and spread the sides of the cake with a thin layer of filling. Spread the remaining filling on the top. Press the chopped walnuts around the side of the cake. Refrigerate the cake for at least 4 hours or overnight. Let the cake stand at room temperature for 15 minutes, then scatter the walnut halves on top before serving.

Make Ahead

The cake can be refrigerated for up to 4 days.