Lots of people have this recipe (or a version of it), but this is really the best version out there. It combines butter and oil for the best flavor/moistness combo.
It's seriously the most moist delicious chocolate cake I've ever had.
One more thing: leftovers in the morning with coffee. Awesome.
Chocolate Sheet Cake
2 cups flour
2 cups sugar
1 stick butter
4 T cocoa powder
1 cup water
1/2 cup oil
1 tsp soda
1/2 cup buttermilk
2 eggs
1 tsp vanilla
Icing:
1 stick butter
3 T cocoa powder
5 T milk
1 pound powdered sugar
1 tsp vanilla
In a large bowl, whisk together the flour and sugar.
In a pot on medium heat, bring to a boil the butter, 4 T cocoa powder, water and oil. Whisk to combine. As soon as it boils, pour it into the flour/sugar mixture and combine slightly - don't completely incorporate it but just enough so that the mixture is just warm (not hot).
Add the soda, buttermilk, eggs & vanilla and mix completely.
Pour onto a greased 10x15 cookie sheet/baking pan) and bake in a 350 degree oven for 15-18 minutes, until just set in the middle. A toothpick inserted in the middle should have just a few crumbs sticking to it.
While the cake bakes, make the icing. In a pot over medium heat, bring to a boil the butter, cocoa & milk. As soon as it boils, remove it from the heat and add the powdered sugar and vanilla. Beat with a hand mixer or a whisk until it is smooth.
When the cake comes out of the oven, cool for just a couple of minutes before adding the icing. The icing will set up and harden slightly as it cools. Cool completely and serve!