01 - Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, tapping out any excess flour.
02 - In a large mixing bowl, sift together the all-purpose flour, unsweetened cocoa powder, granulated sugar, baking powder, baking soda, and salt until evenly distributed 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 2 minutes until the batter is smooth and well combined.
04 - Gradually pour in the boiling water while mixing on low speed. Continue mixing until fully incorporated — the batter will be noticeably thin, which is expected.
05 - Divide the batter evenly between the two prepared cake pans. Bake for 30–35 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 carefully invert onto wire racks to cool completely before frosting.
07 - In a mixing bowl, 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.
08 - Place one cake layer on a serving plate. Spread an even layer of frosting over the top. Place the second layer on top, then frost the top and sides of the entire cake using an offset spatula for a smooth finish.