Preheat oven to 350°. Butter two 8-in. round cake pans. Line bottoms with waxed paper cut to fit. Butter paper, then dust pans with 1/2 tbsp. cocoa.
Whisk remaining 1 cup cocoa, the flour, sugars, cinnamon, soda, cayenne, and salt in a bowl to blend. Add milk, oils, eggs, vinegar, and vanilla; whisk until smooth. Pour into pans, dividing batter evenly.
Bake until a toothpick inserted in each cake comes out clean, about 45 minutes.
Cool cakes in pans for 10 minutes. Loosen cakes from pans with a metal spatula. Turn out each cake onto a plate. Discard waxed paper. Invert cakes onto racks and cool completely. Sift powdered sugar over stencils onto cakes.
Make ahead: Up to 1 day, covered; top with powdered sugar after unwrapping.
Note: Nutritional analysis is per serving.