Preheat oven to 350 degrees, with racks in upper and lower thirds. Cut 2 rounds of parchment paper to line bottoms of 2 (10-by-2-inch) round cake pans. Butter pans; line with parchment. Butter parchment. Dust pans with cocoa; tap out excess.
Sift cocoa, flour, sugar, baking soda, baking powder, and salt into bowl of an electric mixer. With mixer on low, add eggs, buttermilk, the hot water, oil and vanilla; mix until smooth, about 2 minutes.
Divide batter evenly between pans. Bake, switching positions of pans halfway through, until a cake tester comes out clean, about 1 hour.
Let cool in pans on wire racks 20 minutes. Run a knife around edges of cakes; unmold. Let cool completely on racks.
Place 3 cups buttercream in each of 2 bowls; stir yellow food coloring into one, and red food coloring into the other. Transfer each to a pastry bag fitted with a ribbed tip (such as Ateco #48).
Trim tops of cakes to be level. Spread a thick layer of white buttercream evenly on top of one layer. Add second layer, cut side down. Frost top and sides of assembled cake with a thin layer of white buttercream. Refrigerate cake 30 minutes. Smooth final layer of white buttercream on top of cake. With the ribbed side of tip, pipe alternating lines of red and yellow buttercream on sides of cake, and press candies around top and bottom edges.