01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans, then line the bottoms with parchment paper for easy release.
02 - In a large mixing bowl, whisk together the all-purpose flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt until evenly distributed.
03 - Add the 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 - Gradually pour in the boiling water while mixing on low speed. Continue until fully incorporated — the batter will be noticeably thin, which is expected.
05 - Pour the batter evenly between the two prepared cake pans, tapping gently on the counter to release any trapped air bubbles.
06 - Bake on the center rack for 30 to 35 minutes, or until a toothpick inserted into the center of each layer comes out clean.
07 - Allow the cakes to cool in their pans for 10 minutes, then invert onto wire cooling racks and let them cool completely before frosting.
08 - Beat the softened butter with an electric mixer until light and creamy. Gradually blend in the powdered sugar and cocoa powder. Add the milk and vanilla extract, then beat on medium-high speed until the frosting is smooth and fluffy.
09 - Place the first cake layer on a serving plate. Spread an even layer of frosting over the top, then place the second layer on top. Frost the sides and top of the cake, using a spatula to create a smooth or decorative finish.