Preheat oven to 450°F. Line a large baking sheet with parchment paper or butter a cast iron pan and set aside.
In a mixing bowl, whisk together all the dry ingredients. Toss cold butter cubes 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, pea sized bits of butter mixed into the flour.
ALTERNATIVE FOOD PROCESSOR INSTRUCTIONS: Measure all of your dry ingredients into the bowl of your food processor. Pulse a few times to mix. Add cold butter to the dry ingredients and pulse 5-6 times, or until most of the butter is in pea sized, or smaller, pieces. Pour into a mixing bowl and proceed with the rest of the instructions.
Make a well in the center of the mixture and add the cold buttermilk. Gently bring the wet and dry together until you have a fairly uniform, loose dough. The flour won't look all mixed in and that's okay. It will come together.
Dump the butter/dry ingredient mix out onto a lightly floured surface. Using your hands, gently bring it everything 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 inch thick. Using your bench scraper or a knife, cut into square biscuits and place each piece on your parchment lined baking sheet. Place sheet pan in the freezer for 10-15 minutes before baking. (If you're baking your biscuits in a cast iron pan, you'll want to freeze your biscuits on a sheet pan (or other stable surface like a cutting board. Don't freeze the cast iron! Icy cold cast iron pan = delayed cook time!) Round biscuits: Using a biscuit or cookie cutter, cut out your biscuits. Do not twist the cutter when cutting them out! That will effectively seal the edges of the biscuits, keeping them from fully rising. Just press straight down and lift straight up. Gather your scraps as you go and gently re-form them together and cut as many biscuits as you can. Place in freezer as above.
Remove biscuits from the freezer. Brush the tops of the biscuits lightly with an egg wash and sprinkle with flaky salt, if desired.
Bake in a 450° oven for 15-18 minutes, or until golden brown.