Preheat oven to 350°F.
Prepare two round 9-inch by 1 1/2-inch cake pans: grease, line bottom with parchment or wax paper, and then grease again and flour. (If you only have 2-inch high pans, either do 2/3 the recipe for 1 layer or 1 1/3 the recipe for 2 layers). Crisco spray with flour also works well.
In a medium bowl, whisk together the cocoa and boiling water until smooth. Cool to room temperature.
In another bowl, lightly combine the eggs, 1/4 of the cocoa mixture, and vanilla.
In a large mixing bowl, combine the remaining dry ingredients and mix on low speed for 30 seconds. Add the butter and remaining cocoa mixture. Mix on low speed until the dry ingredients are moistened. Increase to medium speed (high speed if using a hand mixer) and beat for 1 1/2 minutes to aerate and develop the cake's structure. Scrape down the sides.
Gradually add the egg mixture in 3 batches, beating for 20 seconds after each addition to incorporate the ingredients and strengthen the structure. Scrape down the sides.
Scrape the batter into the prepared pans and smooth the surface with a spatula. The pans should be about 1/2 full. Bake 20 to 30 minutes or until a tester inserted near the center comes out clean and the cake springs back when pressed lightly in the center. It may take a little longer than 30 minutes.
Let the cakes cool in the pans on racks for 10 minutes. Loosen the sides with a small metal spatula and invert onto greased wire racks. To prevent splitting, reinvert so that tops are up and cool completely before wrapping airtight or frosting.