Preheat the oven to 325 degrees F. Spray one 8-inch round cake pans with nonstick spray. Line the bottom of each pan with parchment paper, then spray the parchment paper. 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, oil, eggs, water and vanilla. Whisk until combined.
With the mixer on low speed, slowly add the wet ingredients to the dry. Mix just until combined and then scrape down the sides and bottom of the bowl and mix again briefly (only about 20 seconds).
Pour the batter into the prepared pan. Bake for 28 to 30 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs on it.
Cool in the pan for 10 to 15 minutes, then turn them out onto a cooling rack and cool completely. (I like to spray my cooling racks with nonstick spray to ensure non of the cake sticks to the racks).
Divide the cake layer, horizontally. Wrap the entire cake layer in plastic wrap and freeze until ready to use.