preheat oven to 400 degrees
grease a 18-by-12 inch sheet pan
combine the flour and sugar in a large bowl
combine the water, butter, and cocoa in a small saucepan and bring to a boil. Pour over the flour mixture
in a small bowl, whisk the buttermilk, eggs, baking soda, and vanilla together. Add to the batter and blend thoroughly by hand. Don't use an electric mixer
pour into the baking sheet and bake for 20 minutes or until a cake tester comes out clean