Position your oven rack in the center and heat your oven to 450°F. Take your cast iron skillet and generously coat the bottom and sides with about a tablespoon of butter or shortening. Set this aside while you prepare the dough.
Sift your flour into a large mixing bowl to ensure a light texture. Add the baking powder, baking soda, salt, and sugar directly to the flour. Use a whisk to thoroughly blend these dry ingredients together – this step is crucial for even leavening and tall, uniform biscuits.
Scatter the cold butter cubes and shortening over the flour mixture. Using a pastry cutter, two forks, or your fingertips, work the fats into the flour until the mixture resembles coarse meal with pea-sized lumps of fat still visible throughout. These fat pockets create steam during baking, which gives you those beautiful flaky layers.
Create a well in the center of your flour mixture and pour in the cold buttermilk. Using a fork, gently stir from the outside in, folding the flour into the liquid. Stop mixing as soon as a shaggy, sticky dough forms – the dough should look rough and slightly wet. If dry spots remain, drizzle in additional buttermilk one tablespoon at a time.
Dust your hands generously with flour. Reach into the bowl and pull off a large portion of dough, roughly the size of a tennis ball. Without kneading or compressing, gently shape it into a thick, rustic mound and place it directly in your prepared skillet. Repeat with the remaining dough to form 8 biscuits, arranging them so they just barely touch – this contact helps them rise upward rather than outward.
Brush the tops of your unbaked biscuits with about half of the melted butter. Transfer the skillet to your preheated oven immediately – working quickly keeps everything cold, which is essential for maximum rise.
Bake for 18 to 22 minutes, rotating the skillet halfway through if your oven heats unevenly. The biscuits are done when their tops turn a deep golden brown and they have risen dramatically – at least doubling in height. The edges should appear set while the centers may still look slightly soft.
Remove the skillet from the oven and immediately brush the hot biscuit tops with the remaining melted butter. Allow them to rest in the skillet for 3 to 5 minutes before using a spatula to gently separate and transfer them to a wire rack or serving basket. Serve warm for the ultimate experience.