First, make your cake! Preheat the oven to 350° Fahrenheit and generously grease 4 nine inch round cake pans ( you can also line them with parchment circles to more easy removal) and set aside. For the pumpkin layers: in a medium bowl, combine the egg, oil, milk, pumpkin, and brown sugar whisk until fully combined. Add in the flour, white sugar, spices, baking powder and soda, and salt and whisk to combine. Pour batter into 2 of the greased cake pans.
For the chocolate layers: in a medium bowl, combine the eggs, butter, water, buttermilk, white sugar, and vanilla and whisk until fully combined. Add in the flour, cocoa powder, baking powder and soda, and salt and whisk to combine. Pour batter into the other 2 greased cake pans. Bake for 30-35 minutes or until a skewer inserted into the cakes comes out clean.Let the cakes cool for at least 25 minutes.
For the Buttercream Frosting:
While cakes are cooling, make the buttercream frosting. In a medium bowl, using an electric mixer, beat together the butter and powdered sugar. Add in 1 cup of the powdered sugar at a time to prevent it from flying everywhere. Then add the milk and continue to beat until it reaches your desired spreadable consistency. Add any flavoring you want (example: vanilla extract) and also any food coloring. With the Halloween theme in mind I chose black and orange!
For assembling the cake, layer chocolate, then pumpkin, chocolate, then pumpkin to give the striped effect. Use the buttercream for filling between each of the layers and then to cover the entire cake. Then the best part...decorating!