Peppermint Patty Cake

Preheat oven to 350°. Lightly grease the bottoms of three 8-inch round cake pans. Line bottoms of pans with parchment paper; grease and lightly flour pans.

For cake: In a 2-cup measuring cup, combine coffee and buttermilk. In the bowl of a stand mixer fitted with a whisk attachment, sift together sugar, flour, cocoa powder, baking soda, baking powder and salt. Add eggs, egg yolks and vanilla. Mix on lowest speed until combined. Gradually add coffee mixture, scraping sides of bowl between each addition, making sure the mixture is uniform before adding more of the coffee mixture. Mix in melted butter.

Divide batter among prepared pans. Bake until a toothpick inserted in centers comes out clean, about 30 minutes. Cool in pans 10 minutes. Turn cakes out onto wire racks; peel off paper. Cool completely.

For buttercream: Place egg whites and sugar in the metal bowl of a stand mixer. Set bowl over a pan of simmering water to create a double-boiler. Whisk constantly until sugar is dissolved and mixture reaches 145° for at least 3 minutes or 160° for at least 15 seconds (test with an instant-read thermometer), about 15 minutes total.

Carefully transfer bowl to stand mixer fitted with whisk attachment. Beat on low 1 minute; gradually increase speed to medium-high, beating until mixture is fluffy, glossy and cool, 10 to 12 minutes.

Reduce speed to medium-low. Add softened butter, a few pieces at a time, beating well after each addition. Beat in peppermint extract. Remove whisk attachment; fit paddle attachment. Scrape down bowl with a rubber spatula. Beat on low until very smooth, 1 to 2 minutes. (Mixture may appear curdled while beating but will eventually come together.)

To assemble: Place one cake layer on a serving plate. Spread a generous 3/4 cup buttercream over the top. Repeat with a second cake layer and 3/4 cup buttercream. Add third cake layer. Use about 1 cup of the remaining buttercream to cover the entire cake with a thin layer of frosting to lock in any loose crumbs. Chill cake until set, about 30 minutes. Frost top and sides of cake with remaining buttercream. If desired, decorate with sprinkles.