Preheat oven to 350 Deg F. Butter three 9-by-2-inch round cake pans. Line with parchment rounds and butter the parchment. Dust with cocoa, tapping out excess, set aside.

Sift the cocoa into a medium bowl. Stir in the boiling water until smooth; gradually whisk in milk. Set aside to cool

Whisk together flour, baking soda and salt into a bowl. With an electric mixer on low, beat butter until fluffy, 3 to 5 minutes. Gradually beat in sugar, scraping down sides of bowl twice. Beat in vanilla

Add eggs in 3 batches, beating well after each addition. With mixer on low speed, add flour mixture and reserved cocoa mixture a little at a time in alternate batches, starting and ending with the flour; beat until just combined

Divide batter evenly among prepared pans; smooth tops with offset spatula. Bake, rotating halfway through, until a cake tester comes out clean, 20 to 25 minutes. Transfer pans to wire racks to cool 15 minutes. Turn out cakes onto wire racks to cool completely.

Fill and frost with desired filling and frosting.