We preferred this half shortening-half butter combination, but all of one (even margarine) works well too. If using margarine, use the higher fat varieties or ones labeled "for baking." Shake the carton of buttermilk before measuring.
Combine first 5 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.
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.
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.
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.