While the cookie crusts cool, preheat the oven to 350 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 mint extract.
With the mixer on low speed, slowly add the wet ingredients to the dry. Scraping the bottom of the bowl with a rubber spatula, and mixing for 30 more seconds on medium-low speed.
Pour the batter into the prepared pans (about 16 ounces in each 8-inch or 12 ounces in each 6-inch pan) and bake for 18-20, until a cake tester comes out clean.
Cool in the pans for 10 minutes, then turn them out onto a cooling rack and cool completely.
Wrap in plastic wrap and chill until ready to use. You're still going to want to level your cake layers to ensure a completely even surface for frosting and stacking.