- Preheat the oven to 350 degrees F.
- For the cake: Whisk together the 3/4 cup of sugar, flour, 1/4 cup of cocoa, baking powder and salt in a bowl.
- Whisk in the milk, melted butter and vanilla.
- Stir together until smooth with a wooden spoon or spatula.
- Pour the batter into an ungreased 9-inch square baking pan.
- Use an offset spatula to level into pan.
- For the topping: Whisk together the 1/2 cup of sugars and 1/4 cup of cocoa and sprinkle it evenly over the batter.
- Pour the hot water over the top, resist the temptation to stir it into the batter.
- Bake about 30 minutes, you want the center to bubble and look almost set, almost like an undercooked brownie.
- Take out of the oven and let stand 15 minutes.
- Serve in dessert dishes, spooning sauce from bottom of pan over top.
- Serve with mint chocolate chip ice cream, if desired.