For batter amounts and baking times, see Cook's Note below; this recipe can be multiplied.
Preheat oven to 350 degrees. Butter pans. Line bottoms with parchment paper; butter parchment. Dust with flour, and tap out excess; set aside. Sift together flour, baking powder, and salt in a bowl; set aside.
Put butter and sugar into the bowl of an electric mixer fitted with the paddle attachment. Mix on medium-high speed until pale and fluffy, about 5 minutes. Add eggs, 1 at a time, mixing until combined after each. Mix in vanilla. Reduce speed to low. Add flour mixture in 3 batches, alternating with the milk and cream.
Stir cocoa powder and boiling water in a small bowl until smooth. Stir in 1 cup cake batter. This will make about 1 cup batter; to make larger amounts of chocolate batter, repeat with additional cocoa powder, boiling water, and cake batter using the same amounts.
Fill prepared cake pans by alternating spoonfuls of vanilla and chocolate batters in a checkerboard pattern. Run a skewer through batter in a figure-eight motion to create a marble pattern. Bake until a cake tester inserted into centers comes out clean. Let cool completely in pans on wire racks. Run a knife around edges to loosen, and turn out cakes onto racks.
9-inch, 4 1/2 cups vanilla batter, 1 cup chocolate batter, 50 minutes; 12-inch, 7 cups vanilla batter, 2 1/4 cups chocolate batter, 1 hour; 15-inch, 9 1/2 cups vanilla batter, 3 1/2 cups chocolate batter, 1 hour 20 minutes