Place sourdough starter in large bowl. Add sugar. Dissolve yeast in warm water and add to starter. On pastry board, cut shortening into flour, salt and baking powder until the consistency of cornmeal. Add flour mixture to starter, stirring well. Turn onto floured pastry board and knead, adding more flour if necessary. Roll dough to 1/2" thickness and cut with biscuit cutter. Dip top of each biscuit in melted butter and place in 9" greased cake pan with edges touching. Cover and set in warm place to rise until doubled (approximately 1 hour). Bake at 400° for 20-25 minutes or until golden. Makes 12 biscuits.