Place sugar, melted butter, egg and milk in a large mixing bowl; beat with an electric mixer on medium speed for 2 minutes. Stir in flour, powder and salt. Spread in a greased 8x8 baking pan. Sprinkle with brown sugar; dot with diced butter and sprinkle with cinnamon. Bake at 425 for about 35 minutes, until a toothpick comes out clean. Cool 10 minutes; cut into squares. Serve warm.