Texas Sheet Cake

This recipe is from someone in my ward, so I don't know if mom's differs a bit:

2 cups flour
2 cups sugar
1 1/2 sticks butter
1/2 cup cocoa
1 cup water
1/2 cup buttermilk
1 tsp baking soda
2 eggs, beaten
1 tsp vanilla

Mix together flour and sugar; set aside. Bring to boil butter, cocoa, and water. Pour over flour mixture. Blend buttermilk, soda, eggs and vanilla. Pour into 18x12 pan, greased and floured. Bake at 350 for 20-25 minutes.

Frosting
1 stick melted butter
1/2 cup cocoa
1/2 cup buttermilk
1 box + 1 3/4 cup powdered sugar
1 tsp vanilla

Bring butter, cocoa, and buttermilk to a boil. Add powdered sugar and vanilla and mix with mixer. Frost cake 15 minutes after removing it from the oven.