In a small bowl, stir together the yeast, warm water, and 1/2 teaspoon of sugar. Let this sit undisturbed for about 4 to 6 minutes until it’s foamy. If no foam appears, discard and try again with fresh ingredients. In a large mixing bowl, whisk the flour, baking powder, remaining 3 tablespoons of sugar, baking soda, and salt together. Drop in the cold shortening and butter pieces. Use a pastry cutter or two forks to work these into the flour until the mixture resembles coarse crumbs, with small chunks about the size of peas.
Pour the yeast mixture and buttermilk over the dry ingredients and mix just until the dough begins to come together. Lightly knead in the bowl 5 to 6 times, until the dough forms a cohesive ball. Wrap the dough in plastic and chill for at least 2 hours or up to 3 days.
When ready to bake, preheat your oven to 400°F. Generously butter the bottom and sides of a 12-inch cast-iron skillet. On a floured surface, roll the dough to about 3/4-inch thickness. Cut into 2-inch rounds and arrange them in the skillet with sides touching. Gather any leftover scraps, roll, and cut again to use as much dough as possible. Brush the tops of the biscuits with 2 tablespoons of the melted butter. Bake for 20 to 25 minutes, or until the tops turn a light golden brown. Once out of the oven, brush with the remaining melted butter and sprinkle with flaky sea salt.
For the Spicy Honey Butter, simply mix together all ingredients in a small bowl while the biscuits are baking. Serve the biscuits warm with this sweet and spicy butter at room temperature. Makes 1/2 cup.