Preheat the oven to 350F/180C. Grease the bottoms of three 8x2-inch round cake pans and line with parchment.
In a large bowl, whisk together the flour, cocoa powder, baking powder, and salt. In a large measuring glass with a spout, combine the buttermilk and coffee. Set aside.
I the bowl of an electric mixer fitted with the paddle attachment, beat the butter and brown sugar on medium speed until very light and fluffy, about 8 minutes. Add the vanilla and beat well. Add the eggs one at a time, mixing well and scraping the sides of the bowl with a rubber spatula after each addition.
Remove the bowl from the mixer, and using a rubber spatula, fold in one-third of the flour mixture until just combined. Add half of the buttermilk mixture and fold until just combined. Repeat with the remaining flour and buttermilk mixtures. Fold in the mayonnaise.
In a small bowl, whisk together the baking soda and vinegar, and quickly fold into the batter. Don’t over-mix. Divide the batter evenly among the prepared pans.
Bake the first two layers in the center of the oven until a wooden pick inserted into the center comes out with a few crumbs, 20 to25 minutes. Repeat with the final layer. Let the cake layers cool in their pans on wire racks for 10 minutes. Using a knife, loosen the sides of the cakes and carefully turn them out onto wire racks. Peel off the papers and let cool completely.
The cake layers will keep wrapped tightly in plastic wrap at room temperature for up to 3 days.
Keyword buttermilk, Dutch process dark cocoa powder, hot coffee