Preheat the oven to 350 degrees. Sift together the flour, baking powder, and salt; set aside. Lightly butter the cake pans. Line with parchment paper; butter parchment, and dust with flour, tapping out any excess; set aside.
Beat the butter and 3 cups of sugar in a mixer bowl until light and fluffy, about 5 minutes. Add the eggs one at a time, mixing until incorporated after each. Stir in the vanilla extract.
Add the flour mixture in two batches, alternating with the buttermilk, and starting and ending with the flour. Set aside one-third of the batter. Combine the cocoa (and espresso powder, if using), the remaining sugar, and the boiling water in a small heatproof bowl; using a rubber spatula, stir until smooth. Stir into the reserved batter.
Fill the pans: Use two-thirds vanilla and one-third chocolate (or mocha) batter, adding batter in large spoonfuls in a checkerboard pattern. Run a wooden skewer lightly through the batter to create a marbled effect.
Bake until a cake tester inserted into the centers comes out clean. Transfer to wire racks; cool for 5 minutes. Invert the cakes onto the racks; remove parchment and let cool for 30 minutes.
Approximate batter amounts and baking times for 3-inch-deep round pans: 6-inch: 4 cups, 50 minutes; 8-inch: 6 1⁄2 cups, 55 minutes; 10-inch: 9 cups, 1 hour; 12-inch: 14 1⁄2 cups, 1 hour 10 minutes.
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