1. Stir together first 3 ingredients in a small bowl. Let stand 5 minutes.
2. Stir together flour and next 4 ingredients in a large bowl; cut butter and shortening into flour mixture with a pastry blender or 2 forks until crumbly. Add yeast mixture and buttermilk to flour mixture, stirring just until dry ingredients are moistened. Cover bowl with plastic wrap, and chill 2 hours to 5 days.
3. Preheat oven to 400°. Turn dough out onto a lightly floured surface, and knead 3 or 4 times. Gently roll dough into a 3/4-inch-thick circle, and fold dough in half; repeat. Gently roll dough to 3/4-inch thickness; cut with a 2-inch round cutter. Reroll remaining scraps, and cut with cutter. Place rounds, with sides touching, in a 10- or 12-inch cast-iron skillet or on a parchment paper-lined baking sheet. Brush biscuits with 2 Tbsp. melted butter.
4. Bake at 400° for 15 to 20 minutes or until golden brown. Brush with remaining melted butter, and serve.