Heat oven to 350°F. Grease and flour two 9-inch round baking pans.
In large mixer bowl stir together sugar, flour, cocoa, baking powder, baking soda, and salt.
Add eggs, milk, oil, and vanilla; beat on medium speed of electric mixer for 2 minutes. Stir in boiling water. (Batter will be thin.)
Pour batter evenly into perpared pans. Bake 30 to 35 minutes or until wooden toothpick inserted into the center comes out clean.
Cool 10 - 15 minutes. Remove from pans to wire racks. Cool completely.
Prepare frosting. Spread between layers, over top and sides of cake.
Frosting
In small mixer bowl, beat butter until creamy.
Add powdered sugar and cocoa alternately with milk, beating well after each addition until smooth and of spreading consistency.
Blend in vanilla. Add additional milk, 1 teaspoon at a time, if needed.
Notes
The frosting recipe has been adjusted to be 1.5 times the original recipe from Hershey's. We never had enough frosting with the original recipe. Therefore, DO NOT adjust the frosting recipe quantities unless you want a LOT of frosting.