Preheat oven to 350 degrees. Butter cake pans, dust with cocoa, then tap out excess. Mix cocoa powder and the boiling water in a small bowl until smooth. Let paste cool.Advertisement
Cream butter and sugar with a mixer until pale and fluffy. Scrape down sides of bowl as needed. Add vanilla, then eggs, beating until well combined.
Sift flour, baking soda, and salt into a large bowl. Whisk milk into cocoa paste. With mixer on low, add flour in 3 additions, alternating with cocoa mixture.
Pour batter into prepared pans (see batter amounts, with pan sizes, and cooking times below). Bake cakes until testers inserted into centers come out clean. Let cool in pans on wire racks for 15 minutes. Immediately run a knife around edge of cake to loosen, and invert cakes onto wire racks. Let cool completely.