01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans.
02 - In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
03 - Add eggs, milk, oil, and vanilla to the dry ingredients. Beat on medium speed for 2 minutes until smooth.
04 - Stir in boiling water; batter will be thin.
05 - Pour batter evenly into prepared pans.
06 - Bake for 30 to 35 minutes or until a toothpick inserted in the center comes out clean.
07 - Allow cakes to cool in pans for 10 minutes, then transfer to wire racks to cool completely.
08 - Beat butter until creamy. Sift in powdered sugar and cocoa powder, add salt and vanilla, and beat until combined. Gradually add milk, beating until smooth and spreadable.
09 - Place one cake layer on a serving plate, spread with frosting. Top with second layer and frost top and sides.
10 - Slice and serve.