I found this on the Pioneer Woman blog! She has lots of great recipes and this one I had to try right away! She describes this at the end of her recipe as the best chocolate sheet cake ever! I would have to agree with her!!
In a saucepan, melt:
2 sticks butter
Add 4 heaping tbsp cocoa. Stir together.
Add 1 cup boiling water, allow mixture to boil for 30 seconds, then turn off heat. Pour over flour mixture, and stir lightly to cool.
In measuring cup, pour 1/2 cup buttermilk.
Add:
2 beaten eggs
1 tsp baking soda
1 tsp vanilla
Stir buttermilk mixture into butter/chocolate mixture. Pour into sheet cake pan and bake at 350 degrees for 20 minutes.
While cake is baking, make icing:
Chop 1/2 cup pecans finely. ( I omitted because I didn't have any!)
Melt 1 3/4 sticks butter in a saucepan.
Add 4 heaping tbsp cocoa, stir to combine, then turn off heat.
Add:
6 tbsp milk
1 tsp vanilla
1 lb minus 1/2 cup powdered sugar
Stir together.
Add pecans, stir together, and pour over warm cake.
Cut into squares, eat, and totally wig out over the fact that you've just made the best chocolate sheet cake. Ever. (direct quote from the pioneer woman!)