Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans. In a large bowl, whisk together the dry ingredients: all-purpose flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt.

In another bowl, mix the wet ingredients: vegetable oil, eggs, buttermilk, and vanilla extract until well combined.

Gradually add the wet mixture to the dry ingredients, stirring gently until just combined. Divide the batter into two bowls; color one with black gel icing color.

Spoon alternating dollops of black and plain batter into the prepared pans, creating a marble effect with a knife or skewer.

Bake for 20-25 minutes or until a toothpick comes out clean. Let them cool in the pans for 10 minutes.

Turn the cakes out onto wire racks to cool completely.

Beat room temperature butter until smooth and creamy. Gradually add powdered sugar and cocoa powder, mixing on low, until fluffy.

Level the cooled cake layers as needed. Spread black buttercream on the first layer, add the second layer, and frost the top and sides.

Melt red candy melts, drizzle over the cake, and decorate with themed sprinkles.