Preheat oven to 425°F. Line a large baking sheet with parchment paper or butter a cast iron pan and set aside.
Whisk together the flour, baking powder and baking soda, sugar, and salt.
Add in cold butter that has been cut into cubes. Toss to coat with flour mixture.
Use a finger snapping motion to press the butter and flour together, a process known as "cutting in the butter". You should end up with small and large pieces of butter incorporated into flour with the texture of lumpy sand.
Shake your buttermilk well and pour over the butter and flour mixture. Using a danish dough whisk, or sturdy spoon, mix the buttermilk and flour mixture together.
You will make a shaggy dough, with dry floury bits at the bottom of the bowl. That is fine. It will all combine in the next steps.
Lightly dust your counter with flour and dump the shaggy dough out. Press the dough into a rough square.
Using a bench scraper, or sharp knife, cut the dough into four pieces.
Stack the dough quarters on top of each other and press down, forming another square. Repeat 2 more times, for a total of 3 cut and stacks.
Press or roll out to ½ inch thickness. Using a 3 to 4 inch biscuit cutter or cookie cutter, cut out biscuits. Gather scraps and re-roll to cut more biscuits. You should get 4 to 6 biscuits, depending on the size you choose. Place on a parchment paper lined large sheet pan.
Place the tray of biscuits into the freezer while you preheat the oven to 425°F. This will allow the butter to get cold again, encouraging flakier layers.
When the oven is at temperature, brush biscuit tops with an egg wash made of one egg whisked with one tablespoon of water. Sprinkle with flaky salt, if desired and bake at 425°F for 20 to 25 minutes, or until golden brown.