Igloo Cake

INGREDIENTS

• Unsalted butter, for baking sheets
• 1 1/4 cups unsweetened Dutch-process cocoa powder, plus more, sifted, for baking sheets
• 2 1/2 cups all-purpose flour
• 2 1/2 cups granulated sugar
• 2 1/2 teaspoons baking soda
• 1 1/4 teaspoons baking powder
• 1 1/4 teaspoons salt
• 2 large eggs, plus 1 large egg yolk
• 1 1/4 cups low-fat buttermilk
• 1/2 cup plus 2 tablespoons vegetable oil
• 1 1/4 teaspoons pure vanilla extract
• Vegetable oil, cooking spray
• 1 quart good-quality chocolate ice cream
• 1 1/2 quarts good-quality vanilla ice cream
• White-Chocolate Buttercream
• 4 large marshmallows, halved
• White, pale-blue, and silver sanding sugar, for decorating
• Marzipan Penguins
• Desiccated unsweetened coconut, for decorating