Preheat the oven to 350°F. Coat the sides of two 8-inch round cake pans with butter, and then sprinkle a light dusting of cocoa powder over the butter. Set the pans aside until the batter is ready.
In a medium bowl whisk together the cocoa powder and the boiling water. (If the mixture is too thick, add in 1 tablespoon of hot water until it looks like creamy chocolate frosting. Once it is well combined, allow the mixture to cool for about 2-3 minutes. Once it has cooled, whisk in the milk until it is smooth and creamy, then set aside. (The mixture should be similar to very chocolate hot cocoa, or dark chocolate milk. Be sure to whisk until all of the cocoa powder is dissolved.)
½ cup unsweetened cocoa powder, ⅓ cup boiling or very hot water, ⅔ cup milk
In a medium bowl, whisk the flour, baking soda, and salt. Set aside.
2 cups all purpose flour, ½ teaspoon baking soda, ¼ teaspoon kosher salt
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar together until creamy. Continue mixing on low, adding in the vanilla, then the eggs.
1 cup unsalted butter, 1½ cups granulated sugar, 1 teaspoon pure vanilla extract, 3 large eggs
Add the cocoa mixture to the butter mixture, mixing on medium speed until it is all well combined. Add in the flour mixture in batches, while mixing on low to medium speed. Continue mixing until the batter is smooth and creamy.
Evenly divide the batter into the two prepared pans and bake for about 28-30 minutes, until a toothpick comes out clean or the top of the cake springs back when touched.
Remove from the oven and allow the cakes to cool before frosting.