Preheat the oven to 325 degrees F.
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.
With the mixer on low speed, slowly add the wet ingredients to the dry, scraping the bottom of the bowl with a rubber spatula.
Pour the batter into the pans on top of the graham cracker crust (about 16 ounces in each pan) and bake for 25 to 30 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 15 minutes, then turn them out onto a cooling rack and cool completely.
Wrap in plastic wrap and chill until ready to use.