Texas Sheet Cake

It's been called Texas Brownies, Mississippi Mud Cake, Texas Sheet Cake, etc. Whatever you call it, it's delicious!
Texas Sheet Cake

Chocolate Mix-
1 stick of butter
1 c water
1/2 c shortening
4 T cocoa
Cake Mix-
2 eggs
2 c sugar
1 t cinnamon
1 t baking soda
1 t vanilla
1/2 c buttermilk*
2 c flour
Frosting-
1 stick of butter
4 T cocoa
6 T milk
1 t vanilla
3-4 c powdered sugar, sifted
1 c nuts, in small pieces (opt)

1. Preheat oven to 400*.
2. Place ingredients for Chocolate Mix in a small sauce pan, on low, until melted and well mixed. Take off heat and set aside.
3. Mix Cake Mix in a large bowl. Add Chocolate Mix.
4. Grease and flour a jellyroll pan (with 1/2" sides). Pour batter on pan, spread evenly, and bake for 22-25 minutes until it passes toothpick test.
5. Once cake is out of the oven: melt butter, cocoa, and milk in a small sauce pan, on low. Take off heat, add vanilla and powdered sugar until desired consistency. Mix well.
6. Let cake cool for at least 10-15 minutes before you spread frosting on it. Sprinkle nuts quickly after spreading frosting, if desired.
*To make "buttermilk", combine 1/2 c milk with 1 tablespoon vinegar. Mix well and let sit for 5 minutes before adding to recipe.