Preheat oven to 425°. Stir together cornmeal mix, flour, and sugar in a large bowl; whisk in buttermilk, eggs, and butter. Pour batter into a lightly greased 13- x 9-inch pan. Bake 30 minutes or until golden brown. Remove from oven, invert onto a wire rack, and cool completely (about 30 minutes); crumble cornbread.