Preheat the oven to 400°F and grease a 13x18-inch sheet pan.
In a large bowl, whisk together flour, sugar, and cocoa powder.
In a saucepan, melt water, butter, shortening, and chocolate. Pour into dry ingredients and whisk.
Whisk in buttermilk, baking soda, salt, eggs, and vanilla until smooth. Pour into the pan.
Bake for 20-22 minutes. Allow the cake to cool completely.