For the cake:*
½ cup Dutch-processed cocoa powder, plus extra for dusting pans
1½ cups all-purpose flour
1 tsp. baking soda
½ tsp. baking powder
¼ tsp. salt
1¼ cups boiling water
4 oz. unsweetened chocolate, finely chopped
1 tsp. espresso powder
10 tbsp. unsalted butter, softened
1½ cups packed light brown sugar
3 large eggs
½ cup sour cream, room temperature
1 tsp. vanilla extract
For the filling, frosting and garnish:
8 oz. white chocolate, finely chopped
1 lb. unsalted butter, softened
1 cup confectioners’ sugar
1/8 tsp. salt
4 tbsp. heavy cream
2 tsp. vanilla extract
1¾ finely ground peppermint candies, plus extra whole candies for garnish
*I made three 9-inch cake layers, and increased the amount of cake batter by 50%. I made the regular amount of filling and frosting called for in the recipe, and it was just enough.
To make the cake, preheat the oven to 350° F. Grease the edges of three 8-inch round cake pans. Dust with cocoa powder and tap out the excess. Line the bottom of each pan with parchment or wax paper. Whisk together the flour, baking soda, baking powder and salt together in a large bowl. In a medium bowl, whisk the boiling water, chocolate, cocoa powder and espresso powder together until smooth.
In the bowl of an electric mixer fitted with a paddle attachment, cream together the butter and sugar on medium-high speed until light and fluffy, 3-6 minutes. Beat in eggs one at a time, scraping down the bowl between additions. Beat in the sour cream and vanilla and mix until incorporated. Reduce the mixer speed to low and mix in the dry ingredients in three additions alternately with the chocolate mixture, beginning and ending with the dry ingredients. Mix each addition just until incorporated.
Divide the batter evenly into the prepared pans, and smooth the tops with a spatula. Bake the cakes until a toothpick inserted in the center comes out with a few crumbs attached, 15-20 minutes, rotating the pans halfway through baking. Cool the cakes in the pans for about 10 minutes, then run a knife around the edges, turn the cakes out onto a wire rack and peel off the parchment paper. Turn the cakes right side up and allow to cool completely.
To prepare the filling and frosting, melt the white chocolate in the top of a double boiler until smooth. Set aside and let cool until no longer warm to the touch. In the meantime, beat the butter on medium-high speed in the bowl of an electric mixer about 30 seconds. Add confectioners’ sugar and salt, and beat at medium-low speed, scraping down the bowl once, about 1 minute. Add the cream and vanilla and beat on medium speed just until incorporated, about 10 seconds. Increase the speed to medium-high and beat until light and fluffy, about 4 minutes, scraping down the bowl as needed. Transfer half of the mixture to a separate bowl and stir in the cooled white chocolate until smooth. Add ¾ cup of the ground peppermint candies to the remaining frosting.
To assemble the cake, place one cake layer on a cardboard cake round or cake platter. Spread half of the peppermint frosting over the cake. Top with another cake layer and spread evenly with the remaining peppermint frosting. Top with the final cake layer. Reserve ½ to ¾ cup of the white chocolate frosting in a pastry bag. Frost the top and sides of the cake with the remaining white chocolate frosting. Coat the top and sides of the cake with the remaining crushed peppermints. Use the reserved white chocolate frosting to pipe a decorative border around the bottom edge of the cake, as well as decorative swirls on top. Garnish with reserved whole peppermints as desired.