Mix sugar and flour together in a large mixing bowl.
Heat cocoa, water and butter in pan until boiling.
Pour over dry ingredients.
Add buttermilk, mix with baking soda, then eggs and vanilla.
Pour into sheet pan and bake for 20 minutes at 400 degrees.
Frosting
While the cake is baking bring butter, cocoa and milk to a slight boil in a saucepan. Once boiling remove from heat and stir in sugar and vanilla. Mix well. Spread over hot cake as soon as it comes out of the oven.