Directions

1. Preheat oven to 350 degrees. Butter two 12-by-17-inch rimmed baking sheets. Line bottoms with parchment paper; butter parchment. Dust with cocoa powder, and tap out excess; set aside.
2. Sift cocoa, flour, sugar, baking soda, baking powder, and salt into the bowl of an electric mixer fitted with the paddle attachment. Add eggs and yolk, 1 1/4 cups warm water, the buttermilk, oil, and vanilla. Mix on low speed until smooth.
3. Divide batter between prepared sheets. Bake until a cake tester comes out clean, about 16 minutes. Let cool on wire racks 30 minutes. Turn out cakes onto racks, and let cool completely.
4. Coat an 8 1/2-inch (9-cup) ovenproof bowl with cooking spray. Line with plastic wrap, leaving a 2-inch overhang. Cut one cake layer crosswise into 4-inch-wide strips, and use to line bowl (there should be no gaps). Cut a 9-inch round from remaining cake layer; set aside.
5. One at a time, mix ice creams in the bowl of electric mixer fitted with the paddle attachment until smooth. Spread 1 cup vanilla ice cream over cake in bowl; spread 1 1/2 cups chocolate ice cream on top. Continue layering ice creams, alternating 1 1/2 cups of each flavor, stopping 1/2 inch from rim. Press cake over ice cream. Wrap in plastic; freeze overnight.
6. Turn out cake onto a 9-inch cake board. Freeze 15 minutes. Spread a thin layer of buttercream (about 1/2 cup) over cake, and freeze until firm, about 15 minutes. Transfer remaining buttercream to a pastry bag fitted with a basketweave tip (such as Ateco #898). With smooth side of tip facing you, and starting at top of cake, pipe 1-inch rectangles all over cake. If cake begins to soften, freeze 15 minutes.
7. Using marshmallows, make the outline of the door. Using a smaller basketweave tip (such as Ateco #895), pipe rectangles over marshmallows. Sprinkle sanding sugar all over cake. Freeze until ready to serve, up to 1 day. Attach marzipan penguins to cake with toothpicks, and garnish with coconut "snow."