How to Make It
Preheat oven to 350°F. Coat 2 (9-inch) round cake pans with cooking spray; dust with flour. Beat butter with a hand mixer on medium-high speed until smooth and creamy, about 1 minute. Add sugar, beating until fluffy, about 3 minutes. Add eggs 1 at a time, beating well after each addition.
Stir together flour, cocoa, baking soda, and salt in a bowl; add to butter mixture alternately with hot water and milk, beginning and ending with flour mixture, beating on medium speed after each addition. Stir in vanilla.
Pour batter evenly into prepared cake pans. Bake in preheated oven until a wooden pick inserted in centers comes out clean, 22 to 25 minutes. Cool in pans 10 minutes. Remove layers from pans, and let cool completely on a wire rack, about 30 minutes.