In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, and sugar.
1 & ½ cup all purpose unbleached flour, 2 teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon salt, 1 teaspoon sugar
Add cold, cubed butter to the dry ingredients. Use a pastry cutter or two forks to cut the butter into the flour mixture until it resembles coarse crumbs with pea-sized chunks of butter.
½ cup butter
Pour in the buttermilk and sourdough discard. Stir gently with a wooden spoon or spatula until just combined. The dough should be slightly sticky and lumpy.
⅔ cup sourdough discard, ½ cup buttermilk cold
Place the bowl in the fridge while preheating the oven to 475°F.
Brush the 10-inch cast iron skillet with melted butter then drop ¼ cup scoops of the biscuit dough into the skillet, leaving a little space between each biscuit. You should have 8 biscuits in the skillet.
Place the skillet in the oven and bake for 12-15 minutes, or until the biscuits are golden brown on top and cooked through. Remove the skillet from the oven, brush the biscuits with extra butter, and let the biscuits cool for a few minutes before serving. Enjoy them warm with butter, honey, or your favorite jam.