Preheat your oven to 350°F (175°C). Grease and line three 8-inch round cake pans with parchment paper. In a large mixing bowl, whisk together the dry ingredients: flour, cocoa powder, baking powder, and salt. Beat in the softened butter until the mixture resembles coarse crumbs.

Gradually incorporate the egg whites, sour cream, vegetable oil, and vanilla extract until the batter is smooth and well-combined.

Divide the batter evenly among the prepared pans and smooth the tops. Bake for 32-35 minutes, or until a toothpick comes out clean. Let the cakes cool in the pans for 10 minutes before transferring to a wire rack.

While cooling, place the layers in the freezer for 30 minutes to firm them up for assembly.

In a large mixing bowl, beat the unsalted butter until creamy. Mix in vanilla and peppermint extracts, then gradually add powdered sugar and heavy cream until fluffy.

Level the cooled cake layers, stack them with peppermint buttercream in between, and frost the outside of the cake.

Pipe decorations onto parchment paper, chill until firm, and attach to the frosted cake with dollops of frosting.

Chill the assembled cake for at least 30 minutes before slicing and serving.