1. Preheat oven to 350 degrees. Spray six 9-by-2-inch cake pans with nonstick cooking spray. Line each cake pan with parchment paper and spray parchment.

2. In a medium bowl, whisk cocoa with hot water until smooth. Whisk in sour cream; let cool. Into a medium bowl, sift together flour, baking soda, and salt; set aside.

3. In the bowl of an electric mixer fitted with the paddle attachment, beat butter and sugar on medium speed until light and fluffy, 3 to 4 minutes. Add eggs, one at a time, beating to combine after each; scrape down sides of bowl as needed. Beat in vanilla. With mixer on low speed, add flour mixture in two parts, alternating with the cocoa mixture and beginning and ending with the flour; beat until combined.

4. Divide batter between prepared pans; smooth with an offset spatula. Bake until a cake tester inserted in centers comes out clean, 12 to 14 minutes. Transfer pans to wire racks to cool 15 minutes. Invert cakes onto racks; peel off parchment. Reinvert cakes and let cool completely, top sides up.

5. Meanwhile, fit a pastry bag with a large plain round tip; fill pastry bag with one-third of the buttercream and set aside.

6. Place four strips of parchment paper around perimeter of a cake plate or lazy Susan. Transfer one layer to cake plate and pipe buttercream around the perimeter of the cake. Using about 1 cup remaining buttercream, fill between the buttercream perimeter to create an even layer of filling; sprinkle with some of the crushed peppermint candies. Top with another layer of cake, top side down. Repeat process, filling the cake, piping the buttercream perimeter, creating an even layer with remaining frosting, and sprinkling with peppermint candies, until all the cake layers have been stacked. Spread remaining frosting all over tops and sides of cake; sprinkle top with peppermint candies. Garnish sides of cake plate with any remaining candies; serve.