Preheat oven to 350 degrees. Grease and flour three 8 inch round cake pans. (If you only have two pans, don't try to bake all the batter in only two pans, it's too much.) (Just pour a ⅓ of the batter into each of your two pans, then chill and cover the rest of the batter while the cakes bake. Once they're baked and removed from their pans, clean one of your pans, grease and flour it, then bake the remaining batter in it.)
In a large mixing bowl, add the flour, cocoa powders, baking powder, baking soda, salt and sugars. Whisk and set aside.
In another bowl, add the softened butter, oil, vanilla paste or extract, milk and eggs. (Do not add the coffee or hot water yet.) Whisk well.
Add the liquid ingredients (except for the coffee or hot water) to the dry ingredients and mix with an electric mixer on medium to medium-high until the ingredients are well combined. Be careful not to mix too long or the cake will turn out dense. Stop mixing once the ingredients are well combined.
Scrape down the sides and bottom of the mixing bowl and add the hot coffee (or water). Mix until well combined.
Pour into prepared pans and bake for approximately 35-40 minutes. (See above note if you only have two pans.)
Set on wire racks to cool for about 15 minutes, then turn out the cake layers onto the racks and remove from the pans to cool completely before assembling the cake and adding buttercream frosting.
Once the cake is cooled, add one layer of cake on a cake plate, then add the buttercream frosting. Continue until all three layers are on the cake plate with two layers of filling. Cover the entire outside of the cake in the buttercream frosting.