Stir white sugar and butter chunks together in a bowl with a spoon; add flour and baking powder. Slowly add milk to flour mixture while stirring until batter is just moistened. Spread batter into a 9x13-inch baking dish. Pour melted butter, brown sugar, and cinnamon evenly over the top.
Bake in the preheated oven until golden and set, 20 to 25 minutes.