Preheat your oven to 350 °F (175 °C). Line 3 round 8" cake pans with parchment paper on the bottom and grease the sides.
In a large bowl, stir together the sugar, flour, Dutch-process cocoa, black cocoa powder, baking powder, baking soda and salt. Add eggs, milk, oil and vanilla.
Mix until combined, and then stir in the boiling water. The batter will be quite thin.
Divide the batter between the three pans. That’s 658 grams of batter per pan (675 grams per pan if using Bob's GF flour).
Bake for 26-28 minutes or until a toothpick comes out with some moist crumbs, but no uncooked liquid.
Let cool for 10 minutes in the pans, and then turn out onto a rack to cool completely, about 2 hours. My cakes weren't flat, but not very rounded, either. If you want to, you can level them (I did not).
Unfrosted cake layers can be wrapped in plastic wrap and stored at room temperature for 2 days or refrigerated for 3. They can also be frozen to make the crumb coat tidier. I recommend at least chilling the cake overnight before stacking the layers, but freezing is even better.