In a large bowl, beat the sugar, sugar substitute, butter and vanilla. Combine the flour, cocoa, baking powder and salt; add to sugar mixture alternately with milk. Spread into an 8-in. square baking dish coated with cooking spray.
For topping, in a small bowl, combine the sugars, sugar substitute and cocoa. Sprinkle over batter. Pour water over topping. Bake at 350° for 37-42 minutes or until top is set and edges pull away from sides of dish. Serve warm with ice cream.