How to Make It
Stir together warm water, yeast, and 1 teaspoon of the sugar in a small bowl. Let stand 5 minutes.
Stir together flour, baking powder, salt, baking soda, and remaining 3 tablespoons sugar in a large bowl; cut cold butter and cold 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; chill at least 2 hours or up to 5 days.
Preheat oven to 400°F. Turn dough out onto a lightly floured surface, and knead 3 or 4 times. Gently roll into a 1⁄2-inch-thick circle, and fold in half; repeat. Gently roll to 1⁄2-inch thickness; cut with a 2-inch round cutter. Reroll remaining scraps, and cut with cutter. Place rounds with sides touching in a 12-inch cast-iron skillet or on a parchment paper-lined baking sheet. (If using a 12-inch skillet, place remaining biscuits in a 10-inch skillet or on a baking sheet.) Brush biscuits with 3 tablespoons of the melted butter.
Bake in preheated oven until golden, 15 to 20 minutes. Brush with remaining 3 tablespoons melted butter, and serve.