01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans, ensuring even coverage to prevent sticking.
02 - In a large mixing bowl, whisk together the all-purpose flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt until uniformly blended.
03 - Add the eggs, whole milk, vegetable oil, and vanilla extract to the dry mixture. Beat until the batter is smooth and no lumps remain.
04 - Gradually pour the boiling water into the batter while mixing continuously. The batter will appear quite thin — this is expected and ensures a moist crumb.
05 - Divide the batter evenly between the two prepared cake pans, using a rubber spatula to scrape the bowl clean.
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 rest in their pans for 10 minutes, then invert onto wire racks to cool completely before frosting.
08 - Beat the softened butter with an electric mixer until creamy. Gradually blend in the powdered sugar and cocoa powder, then add the milk and vanilla extract. Beat until light, smooth, and fluffy.
09 - Place one cooled layer on a serving plate and spread frosting evenly across the top. Position the second layer on top, then frost the sides and top of the cake with the remaining frosting.