3 cups all-purpose flour
2 teaspoons baking soda
3/4 teaspoon salt
1 cup cocoa powder plus 2 tablespoons
1 1/2 cups boiling water
3/4 cup whole milk
2 teaspoons vanilla
1 1/2 cups unsalted butter room temperature
1 1/2 cups dark brown sugar firmly packed
1 1/2 cups granulated sugar
6 large eggs
Preheat oven to 325F. Butter two 9×13 pans, and line them with waxed or parchment paper. Butter the paper and dust the pans lightly with cocoa powder or flour. Tap out excess.
In a medium bowl, sift together flour, baking soda and salt. In a small bowl, whisk together cocoa powder with the boiling water until quite smooth and glossy. Pour in the milk, followed by the vanilla and whisk gently to combine the liquids.
In the bowl of a stand mixer fitted with a paddle attachment, cream butter with the sugars until light and fluffy, at least 5 minutes. Stop the mixer a few times to scrape down the sides of the bowl and the paddle.
Stop the mixer and add the eggs, one at a time, beating well between each addition. Scrape down the sides of the bowl to ensure everything is well combined.
With the mixer on the lowest speed, add about a third of the flour, followed by half the cocoa mixture. Add another third of the flour and the remaining cocoa. Finally, finish with the last of the flour, mixing the batter only until just combined.
Stop the mixer and scrape down the paddle. Divide the batter evenly between the two prepared pans.
Place the pans side-by-side on the middle rack in the oven. Bake until a toothpick inserted into the center comes out clean, about 35-45 minutes. Cool cakes for 5 minutes in the pan, then loosen around the edges with a knife and invert the cakes onto wire cooling racks. Remove parchment paper and discard. Cool completely before frosting.
Place one cake on a serving plate or tray. Spread a 1/2 inch layer of frosting evenly over the top. Place the second cake on top. Frost the entire cake and sides, smoothing or swirling the frosting as desired.
Serve at once or refrigerate, loosely wrapped in plastic, for up to four days. To freeze, place the tray in the freezer until solid. Then wrap tightly in plastic wrap and store for up to three months. Thaw overnight in the refrigerator, then bring to room temperature before serving.