1. Combine first 4 ingredients until well blended. Cut in shortening and chilled butter with a pastry blender or a fork until crumbly. Add buttermilk, stirring just until dry ingredients are moistened.
2. Turn dough out onto a well-floured surface; sprinkle with 1/2 cup self-rising flour. Knead 20 to 25 times, adding up to 1/2 cup additional flour until dough is smooth and springy to touch.
3. Pat dough into a 3/4-inch-thick circle (about 8 1/2 inches round). Cut dough with a well-floured 2-inch round cutter, making 12 biscuits. Place on ungreased baking sheets. Knead remaining dough together 3 or 4 times; repeat procedure, making 6 more biscuits. Lightly brush tops with melted butter.
4. Bake at 500° for 9 to 11 minutes or until golden.
Note: For testing purposes only, we used White Lily Self-Rising Soft Wheat Flour.