01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans.
02 - Whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt in a large bowl.
03 - Add eggs, whole milk, vegetable oil, and vanilla extract to dry mixture. Beat on medium speed for 2 minutes.
04 - Stir in boiling water carefully until batter is smooth; batter will be thin.
05 - Evenly distribute batter between the two prepared pans.
06 - Bake for 30 to 35 minutes until a toothpick inserted into the center comes out clean.
07 - Let cakes cool in pans for 10 minutes, then transfer to a wire rack to cool completely.
08 - Beat softened butter until creamy. Gradually add powdered sugar and cocoa powder, followed by milk and vanilla extract. Beat until smooth and fluffy.
09 - Place one cake layer on serving plate. Spread frosting on top, then stack second layer. Cover top and sides with remaining frosting.