For the cake: whisk together the sugar, flour, cocoa, baking powder and salt in a bowl. Whisk in the milk, butter and vanilla. Stir together until smooth with a wooden spoon or spatula.
Pour thr batter into an ungreased 9-inch square baking pan. Use an off set spatula to level into pan.
For the topping: whisk together the sugars and cocoa and sprinkly it evenly over the batter. Pour the hot water over the top. Resist the temptation stir it into the batter.
Bake about 30 mins. You want the center to bubble and look almost set, almost like a undercooked brownie. Take out of the oven and let stand 15 minutes. Serve in dessert dishes, spooning from sauce from bottom of pan over top. Serve with vanilla or mint chocolate chip ice cream, if desired.