Chocolate Sheet Cake

2 cups butter, divided
8 Tbsp. unsweetened cocoa, divided
1 cup boiling water

2 cups all-purpose flour
2 cups sugar
1/4 tsp. salt
1/2 cup buttermilk
2 eggs
1 tsp. baking soda
1tsp. vanilla
1 cup butter
6 Tbsp. milk
1 tsp. vanilla
1 (16-oz.) box powdered sugar

1. Melt 1 cup butter in the microwave; stir in 4 Tbsp. cocoa. Add in boiling water. Stir in flour and next 2 ingredients. Stir together buttermilk and next 3 ingredients. Stir buttermilk mixture into chocolate mixture. Pour into a lightly greased half-sheet cake pan.

2. Bake at 350° for 20 minutes.
3. Meanwhile, melt remaining 1 cup butter in microwave. Stir in remaining 4 Tbsp. cocoa. Stir in milk, vanilla, and powdered sugar. Pour over warm cake. Cool and cut into squares.