Preheat oven to 450°F. Line a large sheet pan with parchment paper or butter 12 inch cast iron pan and set aside.
In a large mixing bowl, whisk together all the dry ingredients.
Toss the butter into the dry ingredients and, using a pastry cutter or your fingers, mix in the cold butter until you have a mixture of flat pieces and crumbly bits of butter mixed into the flour.
Make a well in the center of the mixture and pour the cold buttermilk right in there. Gently work it together until you have a fairly uniform loose dough. It won't look all mixed in and that's okay. It will come together. Have faith!
Dump it all out onto a flour dusted counter or board. Using your hands, gently bring it all together into a rough square.
With a bench scraper or knife, cut into four smaller squares. Stack the pieces of biscuit dough on top of each other. Smush it all down into a square again. Repeat this cutting and stacking procedure 2 more times.
Pat the dough into a rectangle at least ½ to ¾ inch thick. Using a biscuit cutter, or a knife for square biscuits, cut out your biscuits and place on your sheet pan or in your buttered cast iron pan.
Gather your scraps as you go and gently reform them together and cut as many biscuits as you can.
Brush the tops lightly with an egg wash made with one egg whisked with 1 tablespoon of water.
Bake in a 450°F oven for 15-18 minutes, or until they have a beautiful golden brown crust.