Preheat oven to 350 degrees F. Coat two round 8- or 9-inch cake pans with cooking spray and set aside.
In a large mixing bowl, combine the flour, sugars, cocoa powder, baking powder, baking soda and salt. Mix until combined.
Add buttermilk, vegetable oil, eggs and vanilla to bowl and mix until creamy. Divide the batter between the prepared cake pans and bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the centers come out clean. Let cool.
Using a long, serrated knife, cut the rounded part of ONLY ONE of the cakes off, making the top side flat. Place in the center of a serving plate and cover top and sides with Chocolate Buttercream Frosting. Top with the remaining cake and cover totally with frosting. Cut into slices and serve