Brownie Sheet Cake or Texas Sheet Cake (since we're in Texas)

Here's the recipe for that sheet cake I made for the BBQ. I hope you all enjoy!

1/2 c margarine
1/2 c milk
1 c water
1/2 c oil
1/4 c cocoa
2 c flour
2 c sugar
1 t soda, 1/4 t salt
2 eggs
1 t vanilla

Combine margarine, milk, water, oil and cocoa and bring barely to a boil. Remove from heat. Combine dry ingredients, add liquids, eggs and vanilla. Grease a large cookie sheet. Bake at 375 for 20 min. Let cake cool!
FROSTING: 1/2 c margarine, 1/2 c milk, 4 T cocoa, 4 c powdered sugar, and 1 t vanilla. Combine the butter, milk, and cocoa then bring to a small boil. Add sugar and vanilla. Pour onto cake while hot.