Preheat oven to 350 degrees F (175 degrees C). Grease a loaf pan. Mix flour and sugar together in a bowl. Pour beer into a separate large bowl; add flour mixture and stir until moistened. Pour batter into the prepared loaf pan. Bake in preheated oven until nearly set, 40 to 50 minutes. Pour melted butter over the top of the bread and continue baking until a toothpick inserted into the center comes out clean, about 10 minutes more.