01 - Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
02 - In a large bowl, sift together the flour, cocoa powder, sugar, baking powder, baking soda, and salt until well combined.
03 - Add the eggs, milk, oil, and vanilla extract to the dry mixture. Beat on medium speed for 2 minutes until smooth.
04 - Pour in the boiling water and mix until fully combined. The batter will be thin — this is normal.
05 - Divide the batter evenly between the two prepared cake pans.
06 - Bake for 30–35 minutes, or until a toothpick inserted into the center of each cake comes out clean.
07 - Let the cakes cool in the pans for 10 minutes, then turn out onto wire racks to cool completely before frosting.
08 - Beat the softened butter until creamy. Sift in the powdered sugar and cocoa powder, then add the milk and vanilla extract. Beat until light and fluffy.
09 - Spread frosting between the two cake layers and over the top and sides. Smooth with an offset spatula for a clean finish.