Adjust oven rack to center position. Grease two 8-inch round cake pans with gluten-free nonstick cooking spray. Preheat oven to 350℉.
Remove pans from oven. Allow cakes to cool in the pan for ten minutes and then turn out onto a wire rack to cool completely. Prepare chocolate frosting. Fill and frost the cake.
Whisk together powdered sugar, cocoa powder, and salt in a small bowl.
Beat butter for 30 seconds on high speed until light. Stop mixer. Add powdered sugar mix and milk or coffee. Mix, on low speed, until creamy. If frosting seems dry, add additional milk or coffee until smooth and creamy.
This recipe was tested with Bob's Red Mill 1:1 Gluten-Free Flour Blend. Replacing the flour with another brand might change the texture of the cake. Be sure to use a flour blend that contains xanthan gum. If it doesn't, add 1 teaspoon of xanthan gum to the flour. Whisk to combine. Then use as directed.
Line two 12-cavity muffin pans with paper liners. Prepare batters as directed. Drop about 1 tablespoon of vanilla batter into each cupcake cup. Add a tablespoon chocolate batter. Repeat until the cupcake cup is filled about 2/3 full. Swirl the batters together using a toothpick. Bake until a toothpick inserted in the center comes out clean, about 20 minutes.
Grease a 13 by9-inch sheet pan with nonstick cooking spray. Prepare batters as directed. Spread about three-quarters of the vanilla batter into prepared pan. Drop chocolate batter evenly over plain batter. Spoon remaining vanilla batter on top. Swirl batters together gently with knife or spatula for marbled effect.
Bake until cake tester inserted into the center of the cake comes out clean, about 30 minutes.
Coffee enhances the flavor of chocolate frosting. If you like coffee, use it in the frosting recipe. You won't really taste it but the frosting will have a deeper chocolate flavor.