Preheat oven to 350°F.
Cut parchment paper to fit the bottom of two 8-inch cake pans. Spray paper and sides of pans with cooking spray.
Mix together flour, sugar, and baking soda in a large mixing bowl.
Place butter, water, oil, and cocoa power in a medium-sized pan and cook over medium heat until butter is melted.
Stir until well blended and pour over the flour mixture, again, stirring until well-integrated.
Add buttermilk (or sour milk), eggs, and vanilla, and mix well.
Divide the batter evenly between the two prepared cake pans.
Bake for 20-25 minutes, or until a cake tester inserted into the center of the cake comes out clean. Remove from oven and cool completely on a cooling rack.
When cakes are completely cool, remove parchment paper from the bottom of the cakes.
Meanwhile, while the cake is baking, prepare the frosting.