Line a large baking sheet with parchment paper.
Whisk together one egg with one tablespoon of water for your egg wash. 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.
Make a well in the center of the mixture and add the cold buttermilk. Using a dough whisk or a sturdy spatula, 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 dough 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 a floured 2.5 inch biscuit or cookie cutter, cut out your biscuits. Gather your scraps as you go and gently re-form them together and cut as many biscuits as you can. Place in freezer for 15 minutes while you preheat oven to 450°F.
Remove biscuits from the freezer. Brush the tops 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.