Lightly grease large baking sheet and set aside. Preheat oven to 425F. In large bowl, stir together flour, baking powder, baking soda and salt. Use a pastry blender or two table knives to cut in 1/2 cup butter or margarine until mixture resembles course crumbs; set aside. In small bowl, combine starter and buttermilk. Stir into flour mixture until thoroughly combined. Turn out onto lightly floured surface. Gently knead dough for about 30 seconds. Roll out dough 1/2 inch thick. Cut biscuits with 2 1/2 or 3-inch biscuit cutter. On prepared baking sheet, arrange biscuits with sides touching. Brush tops with melted butter. Bake in preheated oven for 12 to 15 minutes or until tops are golden brown. Remove from baking sheet and serve hot. Makes 24 to 30 biscuits.