Preheat oven to 350F/180C. Generously butter two 8-inch cake pans that are at least 2-inches high, and dust with cocoa powder, tapping the ramekins slightly to remove any excess.
In a large bowl, place flour, cocoa powder, baking powder, baking soda, salt, and sugar. Mix until combined. Set aside.
In a medium bowl, mix together eggs, oil, vanilla, buttermilk, and water. Whisk until smooth. Add egg mixture into flour mixture and mix until smooth. Don’t overbeat – batter will be lumpy at first, but mix gently and eventually stir with a whisk just until smooth. The batter will be very thin and liquid. That's ok.
Scrape batter into prepared pans. Bake cakes for about 30-35 minutes (depending on your oven - this might take longer), or until a skewer inserted into the center comes out clean or with just a few moist crumbs. Allow to cool for 10 minutes in the pan before unmolding, then let cakes cool completely on a wire rack.
Assembling the cake: Set one layer on a plate with the flat side facing up (or, if your cake rose too much, cut its rounded top with a knife to make it flat – totally optional). Evenly spread about 1/3 of the frosting over the cake to the edge. Top with the second cake layer, rounded side up. Spread the remaining frosting over the top and sides of the cake.
Store cake in the fridge for up to 3 days. Bring to room temperature before serving.