Preheat oven to 350 degrees Fahrenheit.
Prepare two 9-inch cake pans by greasing them well with 2-3 tsp butter and dust them with 2 tbsp of cocoa powder. Tap out as much of the excess cocoa powder as possible; you only want a very thin layer to coat the inside of the cake pans so it disappears as the cakes bake.
After the cake pans are greased and floured, measure the flour, white sugar, brown sugar, cocoa powder, baking soda, baking powder and sea salt into a large mixing bowl through a sieve or flour sifter. This will help prevent lumps in the dry ingredients.
Once you've pushed the dry ingredients through the sieve, give the mixture a stir with a wire whisk to combine all the ingredients.
Add the buttermilk, vegetable oil, eggs, and vanilla and whisk them together into. the dry ingredients with a whisk and/or a spatula. The batter will appear rough and scraggly; this is expected.
Set your kettle to boil some water. You'll need 1 cup.
Switch to a whisk and very slowly pour the boiling water into the cake batter (you can use hot coffee instead of hot water, if desired). Adding it slowly will bring the temperature of the mixture up slowly and prevent any clumps of batter from being "cooked" by the heat of the water. Be sure to whisk the mixture gently and constantly as you add the water.
Pour the cake batter evenly into your prepared 9-inch cake pans.
Bake at 350 degrees Fahrenheit for about 30-35 minutes. After 30 minutes, if the cakes appear to be done, test the middle of one with a toothpick. If a toothpick inserted into the middle comes out pretty much clean, the cakes are done.
Remove pans from the oven and allow cakes to cool in their pan for at least an hour before removing. This will help the cakes pull away from the sides of the pan naturally and they'll be much easier to get out!
After they've cooled significantly, place a wire rack upside down over the top of each cake pan. Invert the cake pan and the cakes should slip right out, upside down, onto the wire racks. Leave them like this to cool completely.
Once the cakes have cooled completely to room temperature, wrap them in plastic wrap and refrigerate them before frosting.