01 - Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
02 - In a large bowl, sift together the all-purpose flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt until evenly blended.
03 - Add the room-temperature eggs, whole milk, vegetable oil, and vanilla extract to the dry mixture. Beat on medium speed for 2 minutes until the batter is smooth and well combined.
04 - Pour in the boiling water and stir until fully incorporated. The batter will appear thin — this is normal.
05 - Divide the batter evenly between the two prepared cake pans. Bake for 30 to 35 minutes, or until a toothpick inserted into the center of each cake comes out clean.
06 - Allow the cakes to cool in their pans for 10 minutes, then turn them out onto wire racks to cool completely before frosting.
07 - Beat the softened butter until creamy. Sift in the powdered sugar and cocoa powder, then add the milk and vanilla extract. Beat until light and fluffy.
08 - Spread an even layer of frosting between the two cake layers, then cover the top and sides with the remaining frosting. Smooth with an offset spatula.