Preheat the oven to 325 degrees F. Spray three 8-inch round cake pans with non-stick spray, line with parchment paper, and spray again. Set aside.
Combine the flour, sugar, cocoa, baking soda, baking powder, and salt into the bowl of an electric mixer fitted with a paddle attachment and mix on low speed until combined.
In another bowl, combine the buttermilk, water, oil, eggs, and vanilla.
With the mixer on low speed, slowly add the wet ingredients to the dry, scraping the bottom of the bowl with a rubber spatula.
Divide the batter between the three pans (about 16 ounces in each) and bake for 20 to 25 minutes, or until a toothpick inserted in the center of the cake comes out with a few moist crumbs on it.
Cool in the pans for 10 minutes, then turn them out onto a cooling rack and cool completely.
Wrap in plastic wrap and freeze until ready to use.