Chocolate Sheet Cake

This recipe is amazing.  The Hammon family only recognizes chocolate cake as actual cake so I am always on the look out for a different version for birthday's.  This recipe is from PW, she knows her stuff.

2 eggs
1 tsp baking soda
8 Tbsp cocoa
2 C flour
1/4 tsp salt
2 C sugar
2 tsp vanilla
3 3/4 sticks butter
1/2 C buttermilk
6 Tbsp milk
1 C water
1 lb powdered sugar

Cake:

Melt 2 sticks of butter in sauce pan (while it's melting boil 1 c. water); Add 4 heaping Tbsp cocoa powder and mix well; add boiling water and allow to simmer for 30 seconds; turn off heat and set aside; combine 2 c. flour, 2 c. sugar and 1/4 tsp salt; pour the chocolate mixture in to flour mixture and stir together slightly; in oversize measuring cup pour 1/2 c. buttermilk, 2 beaten eggs, and 1 tsp vanilla;  stir in 1 tsp baking soda; add milk mixture to cake mixture; mix well;  Pour into commercial baking sheet and spread evenly; Bake at 350 degrees for 20 minutes

Frosting:

In sauce pan melt 1 3/4 sticks butter; add 4 heaping tbsp cocoa powder; allow to cook for 30 seconds; turn off heat; add 6 Tbsp milk, and 1 tsp vanilla; mix together;  add powdered sugar; mix well.  This will be very thin.  Put onto warm cake.