Preheat oven to 350° F. Spray three 6-inch cake pans with cooking spray, and line the bottoms with parchment paper. *Note that the recipe for this batter typically makes three 8-inch layers; you can make 2 extra 6-inch layers or cupcakes with the remaining batter.
In the bowl of a stand mixer fitted with a whisk attachment, add all dry ingredients (cocoa powders, flour, baking soda, baking powder, salt, white sugar, brown sugar) and mix on low-medium speed until combined.
Add the eggs+ yolk, water, buttermilk, oil and vanilla and mix on low-medium until fully incorporated and there are no big lumps. Do not overmix.
Divide batter evenly into three 6-inch cake pans and bake for approximately 25 minutes or until the middle is mostly set. Use a toothpick to test; if it comes out with a few crumbs then it is ready.
Remove from oven and let cool. Transfer to countertop covered with parchment paper by running a knife around the cake edges, then turning the cake pan upside down to release the cake. Once completely cool, you can wrap these in saran wrap and place in a gallon size freezer bag; you can freeze the cake layers for about 2 months. When ready to fill the cake, use frozen cake layers to prevent the cake from crumbling while decorating.
Mint Chocolate Ganache Filling
Add chocolate to mixing bowl and set aside.
Add heavy cream to a saucepan, and heat on medium until it just starts to boil around the edges and center (be careful as this can boil over very quickly).
Remove cream from heat and slowly pour over the chocolate. Let rest for about 5 minutes, then use a whisk to stir the chocolate/cream mixture until fully incorporated and smooth.
Add mint extract and stir until fully incorporated. If using peppermint extract, be sure there is no water in the ingredient listing as this will cause the chocolate to seize (oil is fine).
Let sit at room temperature, or refrigerate, until it is the same consistency as peanut butter. This makes it easy to fill the cake layers.