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, sift together the all-purpose flour, unsweetened cocoa powder, granulated sugar, baking powder, baking soda, and salt until well combined and free of lumps.
03 - Add the room-temperature eggs, whole milk, vegetable oil, and vanilla extract to the dry mixture. Beat on medium speed for approximately 2 minutes until the batter is smooth and homogeneous.
04 - Gradually pour in the boiling water while mixing on low speed. Continue until fully incorporated. The batter will appear thin — this is expected.
05 - Pour the batter evenly between the two prepared cake pans, using a spatula to smooth the surface for uniform baking.
06 - Place pans on the center rack and bake for 30 to 35 minutes. Test doneness by inserting a toothpick into the center of each cake — it should come 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 unsalted butter until light and creamy. Sift in the powdered sugar and cocoa powder, then add the milk and vanilla extract. Beat on medium-high speed until the frosting is fluffy and spreadable.
09 - Place the first cake layer on a serving plate. Spread an even layer of frosting over the top. Position the second layer and frost the top and sides of the entire cake, smoothing with an offset spatula.