Texas Sheet Cake

I have made several variations on this recipe over the past few months, and I think I finally found my "keeper." Next time, however, I am doubling the frosting. Because I like frosting.

Texas Sheet Cake

Cake:
1/2 cup butter
1/4 cup shortening
1 cup water
1/4 cup cocoa
2 cups sugar
2 cups flour
2 eggs
1 1/2 cups buttermilk
1 tsp baking soda
1 tsp vanilla
1 tsp cinnamon (optional)

Preheat oven to 400. In a saucepan, combine butter, shortening, water, and cocoa and bring to a rapid boil. In a bowl combine sugar and flour. Pour saucepan mixture over sugar and flour and mix well. Add buttermilk, eggs, vanilla, soda, and cinnamon. Beat well. Pour into a greased 10x15 jelly roll pan. Bake for 20 minutes.

Frosting:
1/2 cup margarine
1/4 cup cocoa
1 pound powdered sugar
1 tsp vanilla
4-6 T milk
chopped nuts (optional)

In a saucepan, bring butter and cocoa to a boil. In the bowl of your mixer, combine butter mixture, powdered sugar, and vanilla. Mix well, adding milk until desired consistency and frosting is smooth and creamy. Frost cake while it is still warm.