Preheat oven to 350℉ (175℃).
Sift together the first six dry ingredients (sugar, flour, cocoa powder, baking soda, baking powder and salt) into a large bowl and set aside.
In a measuring jug, whisk together the next four ingredients (buttermilk, melted butter, eggs, and vanilla), and pour into a mixing bowl.
Add the sifted dry ingredients onto the liquid in the bowl.
Beat at low speed on a stand mixer, whisk by hand or use a hand mixer; scrape down the sides with a spatula, then mix at high speed for one minute.
Return the speed to low, and pour in boiling-hot water and mix well, using spatula to scrape the sides so the thin batter is evenly mixed.
Divide the mixture evenly into the 3 prepared pans. Bake 2 pans in the upper third of the oven, and one in the lower third.
After about 13 minutes, reverse the trays, and turn them 180 degrees so they bake evenly. Continue to bake for another 7 to 12 minutes, or until a tester comes out clean
Cool cakes in pans about 15 minutes, then run a thin knife around the edges and remove from the pans. Peel off the paper and allow to cool completely on a metal rack.