Fluffy Coffee and Walnut Cake With Mocha Buttercream

Preparation

1. Cake

   Step 1

   Place a rack in middle of oven; preheat to 325°. Lightly butter three 8"-diameter cake pans and line bottoms with parchment paper rounds. Toast 2 cups (250 g) raw walnuts in a single layer on a rimmed baking sheet, tossing halfway through, until golden brown, 12–14 minutes. Let cool. Set aside 1 cup walnuts for serving.

   Step 2

   Meanwhile, cook ¾ cup (1½ sticks) unsalted butter in a small saucepan over medium heat, stirring often, until it foams, then browns, 6–8 minutes. Remove from heat and immediately stir in ½ cup plus 2 Tbsp. vegetable oil or refined coconut oil, melted. Set brown butter mixture aside.

   Step 3

   Pulse remaining 1 cup toasted walnuts in a food processor until coarsely ground (it should be the texture of panko). Transfer to a large bowl and whisk in 2 cups (250 g) all-purpose flour, 2¼ tsp. baking powder, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and ½ tsp. baking soda.

   Step 4

   Process 4 large eggs, 1¼ cups (250 g) sugar, 2 tsp. vanilla extract, and 1 tsp. instant espresso powder in food processor (no need to clean bowl) until mixture is lightened in color and thickened, about 30 seconds. Stir 2 Tbsp. whole milk into reserved brown butter mixture. With motor running, gradually stream brown butter mixture into egg mixture and process until combined. Add dry ingredients and process just until a smooth batter forms. Divide batter among prepared pans.

   Step 5

   Bake cakes, rotating pans front to back after 15 minutes, until puffed and just starting to pull away from the sides of pans and a tester inserted in the centers comes out clean, 20–22 minutes. Transfer pans to wire racks and let cakes cool 15 minutes. Run a small knife around edges of each cake to release from pan if needed. Turn out cakes onto racks and remove parchment paper; discard. Turn cakes right side up and let cool completely.

2. Buttercream and assembly

   Step 6

   Combine 8 large egg whites, 2¼ cups (450 g) sugar, ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and ¼ tsp. cream of tartar in the bowl of a stand mixer. Set over a medium saucepan filled with 1½" of simmering water (bowl should not touch water). Heat, stirring and scraping often with a heatproof rubber spatula, until sugar is dissolved and an instant-read thermometer registers 185°, 10–15 minutes.

   Step 7

   Fit bowl onto stand mixer fitted with whisk attachment and beat, starting on low speed and gradually increasing to high, until meringue is shiny and stiff, 8–10 minutes. Beat in 1 tsp. vanilla extract.

   Step 8

   With mixer on medium speed, add 3 cups (6 sticks) unsalted butter, room temperature, cut into pieces, a piece at a time, mixing until incorporated before adding more. Once all the butter has been added, increase speed to high and beat 1 minute. Transfer buttercream in bowl to refrigerator and chill 30 minutes.

   Step 9

   Fit bowl with buttercream back onto stand mixer and add ¼ cup (21 g) unsweetened cocoa powder, sifted if needed, and 2 tsp. instant espresso powder. Beat on medium-low speed, scraping down sides as needed, until combined.

   Step 10

   Using a large serrated knife and long sawing strokes, remove tops of cakes to level if needed. Place a cake layer, right side up, on a cake stand or large plate. Using an offset spatula, spread ¼ cup buttercream over cake, working all the way to the edges. Place second cake layer, right side up, on top; spread another ¼ cup buttercream over cake, working all the way to the edges. Place third cake layer, cut side down, on top, and press lightly to adhere. Spread top and sides of cake with ½ cup buttercream in a thin, even layer. Chill, uncovered, at least 1 hour and up to 12 hours. (If chilling cake longer than 1 hour, cover remaining buttercream in bowl and chill. Bring to room temperature and beat again before using.)