In a large saucepan, bring to a boil margarine, water and cocoa powder.
Remove from heat and cool slightly.
Add in flour, eggs, baking soda, sugar and sour cream to the saucepan.
Mix together well.
Pour batter into a buttered and floured 10”x15”x2" baking pan and bake for 22 minutes.
For the frosting: in a saucepan, bring to boil margarine, milk and cocoa powder.
Remove from heat and add in chopped nuts, powdered sugar and vanilla.
Beat well until combined and immediately pour over warm cake.
Let cool until frosting is set.
I use margarine in this recipe because that is what Grandma used. Butter will work but I found after testing that margarine actually produces a moister cake.
This way this Texas Sheet Cake is made is a more traditional recipe. Some of the ingredients used were popular in baking during the 1950's when this recipe became really popular. In a lot of early 20th century baking, you'll see a reference to using "Oleo." This was just a name brand for margarine.
This is made in a jelly roll pan which is 10"x 15". It will also work in a larger 12"x17" but the cooking time needs to be lessened so it won't over-bake.
Make sure to butter and flour your pan so this cake can release nicely.
A higher sided pan is recommended so the frosting doesn't go over the sides. I have a 10"x 15"x 2" pan that works perfect. If your frosting does overflow a bit, just gently push it back over the cake with a spatula.