Chocolate Sheet Cake II

Preheat oven to 350 degrees F (175 degrees C). Grease and flour an 11x17 inch sheet cake pan.
In a large bowl, sift together the flour, sugar and baking soda. Stir in the eggs, vanilla and buttermilk. Set aside. In a saucepan combine butter, oil, cocoa and water. Bring to a boil and stir until smooth. Slowly blend into the flour mixture. Pour into prepared pan.
Bake in the preheated oven for 15 to 20 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.