Preheat a cast-iron skillet in the oven to 450°F.
Whisk together the flour, baking powder, baking soda, and salt.
Place a box grater over the mixing bowl containing the flour mixture and grate the frozen stick of butter using the large holes.
Chop up any remaining butter that’s too small to grate.
Cut the butter in with a pastry blender until you see clumps forming.
Use your fingers to sift through the flour, making sure everything is evenly distributed and breaking up any clumps that are too large.
Pour in the cold buttermilk and stir until the dough starts to form and no longer sticks to the sides of the bowl.
Place the dough on a lightly floured surface and begin forming a square.
Using a dough cutter/scraper, slice the dough into fourths, stack them, and press them down to form another square.
Repeat this step 4-5 times until the dough becomes springy when pressed down.
Press the dough down to your desired thickness, but no less than 1 inch.
Using a 3-inch biscuit cutter, cut out 5-6 biscuits and set them aside.
Carefully remove the cast-iron skillet from the oven, then melt about 3 tablespoons of butter in it.
Place the biscuits in the center of the skillet so they are touching eachother.
Brush the tops of each biscuit with a little more buttermilk, then increase the oven temperature to 500°f.
Place the skillet in the oven and bake for about 15-18 minutes, until the biscuits are golden brown and cooked through.
Once baking is complete, immediately transfer the biscuits to a tea-towel-lined bowl and serve hot.