01 - Preheat oven to 350°F (175°C). 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, unsweetened 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 free of lumps.
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 result.
05 - Divide the batter evenly between the two prepared cake pans, smoothing the tops with a rubber spatula.
06 - Bake on the center rack for 30–35 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 light and creamy. Gradually add the powdered sugar and cocoa powder, alternating with splashes of milk and vanilla extract, until the frosting is smooth, fluffy, and spreadable.
09 - Place one cooled layer on a serving plate and spread a generous layer of frosting over the top. Position the second layer on top, then frost the sides and top of the cake evenly. Serve at room temperature.