Combine yeast and warm water, stirring to dissolve; let stand 5 minutes or until bubbly. Set aside.
Sift together dry ingredients. Cut in shortening with a pastry blender until mixture resembles coarse meal; add yeast mixture and milk, and mix well. Turn dough out onto a floured surface; knead 4 to 5 times.
Roll dough to 1/4-inch thickness; cut 18 biscuits using a 2- inch biscuit cutter and an additional 18 biscuits using a 1 1/4-inch biscuit cutter. Place larger biscuits 2 inches apart on an ungreased baking sheet; top each with a smaller biscuit. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in bulk. Bake at 450° for 8 minutes. Brush tops with melted butter before serving.