Combine first 3 ingredients in a large bowl; cut in shortening with pastry blender until mixture is crumbly. Add buttermilk, stirring with a fork until dry ingredients are moistened. (Dough will resemble cottage cheese and be sticky.)
Turn dough out onto a heavily floured surface, and knead lightly 4 or 5 times.
Roll dough to 3/4-inch thickness; cut with a 2 1/2-inch round cutter. Stack two baking sheets, one on top of the other. Lightly grease top baking sheet.
Place biscuits on top baking sheet, and bake at 450° for 10 to 12 minutes or until lightly browned. Brush with butter.