Preheat the oven to 350 F (180 C) degrees.
Line the bottom of three 8-inch round cake tins with rounds of parchment paper. Lightly grease the sides and sprinkle with cocoa powder.
In a very large bowl, whisk together the flour, cocoa powder, baking soda, baking powder and salt. If the cocoa powder is very lumpy, sift it first.
In a separate large bowl, whisk together the oil, sugar, eggs, vanilla extract and buttermilk.
Carefully whisk the wet ingredients into the dry ingredients. This can also be done with an electric mixer on low speed. Ensure there are no lumps - the batter will be thick.
Very carefully whisk (or beat with an electric mixer) the boiling water into the cake batter. It will be quite a bit thinner now.
Divide the cake batter evenly between the 3 pans. You can weigh the pans to ensure they're the same size.
Bake in the preheated oven for 18-22 minutes, or until an inserted toothpick comes out clean and the cakes are starting to pull away from the sides of the pans.
Remove from the oven and cool the cakes in their pans for at least 30 minutes, then gently invert the cakes and continue cooling on a wire rack.