Preheat oven to 350 degrees. Butter pans. Line with parchment cut to fit. Butter parchment. Dust with cocoa. Tap out excess.
Stir together cocoa and boiling water. Add milk, and whisk until smooth.
Beat butter with a mixer on medium speed until light and fluffy. Add sugar in a slow stream, and beat well, about 3 minutes. Beat in vanilla. Add eggs, one at a time, beating well after each addition.
Sift together flour, baking soda, and salt. Reduce mixer speed to low. Add the flour mixture in 3 additions, alternating with the cocoa mixture.
Pour batter into pans, and bake until a few crumbs remain on testers (batter amounts and cooking times follow). Let cool in pans on wire racks for 45 minutes. Unmold, and remove parchment. Let cool completely on wire racks.
Batter Amounts and Cooking Times for 2-Inch-Deep Pans
5-inch: 1 1/4 cups batter, 45 minutes
7-inch: 2 1/4 cups batter, 45 minutes
9-inch: 4 cups batter, 50 minutes
11-inch: 6 cups batter, 50 minutes
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