1. Preheat oven to 300 degrees. Butter three 8-inch round cake pans. Line bottoms with parchment paper; butter parchment, and sprinkle with flour, tapping out excess.

2. In the bowl of an electric mixer fitted with the whisk attachment, beat yolks and 1/4 cup sugar until thick and pale, about 3 minutes. Transfer to a bowl. In a separate bowl, toss together nuts, chocolate, zest, baking powder, and salt. Sprinkle over yolk mixture; do not mix in.

3. Whisk egg whites in the heatproof bowl of an electric mixer set over a pan of simmering water until whites are warm to the touch. Attach bowl to mixer fitted with clean whisk attachment; beat on high speed until soft peaks form. Beat in remaining 3 tablespoons sugar until stiff and glossy peaks form.

4. Fold egg-white mixture into egg-yolk mixture, one-third at a time. Divide batter evenly among prepared pans, and spread with an offset spatula to smooth the tops. Bake until springy to the touch, about 20 minutes. Transfer to a wire rack to cool, about 10 minutes. Run a knife around edge of each cake to loosen, and invert cakes onto wire racks, removing parchment paper. Let cool completely.

5. Place one cake layer on a turntable or flat work surface. Spread 1 1/4 cups buttercream evenly over the top. Place second cake layer on buttercream; evenly spread 1 1/4 cups buttercream over the top. Place final cake layer on top; evenly spread remaining 1/2 cup buttercream on top. Refrigerate at least 30 minutes while you make the ganache.

6. Transfer cake to a wire rack set over a rimmed baking sheet. Pour ganache over top of cake; use an offset spatula to smooth top and sides. Place cake in the refrigerator, and let ganache set, at least 30 minutes, before serving.