Preheat oven to 350 degrees.
To prepare the topping: In a medium-size bowl, stir together the flour, the sugar, and the baking powder. Add the milk and the butter and beat until smooth.
To prepare the filling: In a large saucepan, combine all the ingredients. Cook and stir over medium heat until mixture is thickened and bubbly, about 5-10 minutes. Pour into an ungreased 13x9-inch glass baking dish. Spoon the topping over the filling and spread carefully and evenly with a rubber spatula. Sprinkle with cinnamon sugar. Bake 30-40 minutes or until topping is lightly golden and a cake tester inserted into the center of topping comes out clean.
The Magnolia Bakery Cookbook, Simon & Schuster