Preheat oven to 350 degrees. Generously spray three 5 3/4-by-3-by-2-inch loaf pans with cooking spray; set aside. Sift together the flour, baking powder, and salt; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy, about 5 minutes. Add eggs, one at a time, beating until combined after each addition and scraping down the sides of the bowl as needed. Mix in vanilla. Add flour mixture, alternating with the buttermilk, beginning and ending with the flour. Set aside 1/3 of the batter.
In a bowl, mix cocoa and 1/4 cup boiling water with a rubber spatula until smooth. Add the cocoa mixture to the reserved cake batter; stir until well combined.
Spoon batters into the prepared pans in 2 layers, alternating spoonfuls of vanilla and chocolate to simulate a checkerboard. To create marbling, run a table knife (or wooden skewer) through the batters in a swirling motion. Place pans on a baking sheet.
Bake, rotating the pan halfway through, until a cake tester comes out clean, about 35 minutes. Transfer pan to a rack to cool 10 minutes. Turn out cakes from pans and cool completely on the rack.
Cakes can be kept in an airtight container at room temperature up to 3 days.