Preheat oven to 400 degree F. Grease a 13" by 9" pan with butter or line with parchment paper.
In a pan over medium heat, combine butter, cocoa powder and oil. Bring to a boil and remove from heat.
In a large mixing bowl, combine granulated sugar, eggs, buttermilk and vanilla. Slowly whisk in the butter mixture. Add in dry ingredients and stir just until combined.
Pour into your prepared baking dish and bake for 20 minutes. Remove from oven and cool.