Stir together first 3 ingredients in a small bowl. Let stand 5 minutes.
Stir together flour and next 4 ingredients in a large bowl; cut butter and shortening into flour mixture with a pastry blender until crumbly. Add yeast mixture and buttermilk to flour mixture, stirring just until dry ingredients are moistened. Cover bowl with plastic wrap, and chill dough 2 hours ... to 5 days! (good step for planning ahead here)
Preheat oven to 400*F. Turn dough out onto a lightly floured surface; knead 3 or 4 times. Gently roll dough into a 3/4-inch-thick circle, fold dough in half; repeat. (see our Tips in the Blog section on 'Rolling Dough')
Gently roll (or pat) dough to 3/4-inch thickness; cut with a 2-inch round 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
Bake at 400*F for 15 to 20 minutes or until golden brown. Brush with remaining melted butter, and serve.
Serving Size 1