Preheat oven to 350F
Grease three 6-inch cake pans and dust with cocoa powder. Line bottoms with parchment then set aside.
Place all dry ingredients into the bowl of a stand mixer fitted with a paddle attachment. Stir on low to combine.
Dissolve the coffee in the hot water then set aside.
In a medium bowl whisk together the oil, buttermilk, eggs, and vanilla.
Temper the egg-milk mixture by slowly whisking in the hot coffee (you don't want to cook the eggs!)
Add the coffee-egg mixture to the dry ingredients and mix on medium for 2-3 mins. The batter will be very thin, that's okay!
Pour the batter evenly into the prepared pans. (I used a kitchen scale to ensure the batter is evenly distributed.)
Bake for 25-30 minutes, rotating the pans in oven halfway through. Bake until a cake tester comes out clean.
Cool 10 minutes in the pans then turn out onto a wire rack to cool completely.