Heat oven to 350 degrees Fahrenheit. Lightly grease a 9-inch square or 2-quart round baking dish with cooking spray. Set aside.
For the cake: Whisk together the sugar, flour, cocoa, baking powder, and salt. Stir in the milk, melted butter, and vanilla. Mix until combined. Spread batter into the prepared pan.
For the hot fudge pudding sauce: Stir together the sugar, brown sugar, and cocoa powder. Sprinkle mixture evenly over the top of the cake batter. Pour HOT water over the top. DO NOT STIR.
Bake at 350˚F for 35-40 minutes or until the center is almost set. Remove from oven and let cake rest for 15 minutes, so the sauce will cool slightly and thicken. Serve warm with a scoop of vanilla ice cream, if desired.
Notes
*Heat hot water for cake with a tea kettle, pot on the stove, or in the microwave until simmering.