Stir the yeast and 1 teaspoon sugar in the warm water. Let stand for five minutes.
In a large bowl, combine all dry ingredients, including remaining sugar. Cut in the butter and shortening with two forks or a pastry blender until texture resembles coarse crumbs.
Add yeast mixture and buttermilk to flour mixture. Stir until dry ingredients are moistened. Cover with plastic wrap and chill at least two hours.
Preheat oven to 400 degrees. Turn dough out onto a lightly floured surface and knead three to four times. Gently roll into a ½-inch-thick circle and then fold in half. Repeat rolling to ½-inch-thickness and cut with 2-inch round cutter. Place rounds with sides touching in a 10- to 12-inch cast-iron skillet. Brush biscuits with two tablespoons melted butter.
Bake in preheated oven until golden, 15 to 20 minutes. Brush baked biscuits with remaining melted butter.
