Preheat your oven to 450°F. Lightly grease a 10-inch cast-iron skillet with butter, or line a baking sheet with parchment paper.
In a large mixing bowl, whisk together the all-purpose flour, sugar, baking powder, baking soda, and kosher salt until well combined.
Add the cold cubed (or shredded) butter to the flour mixture. Using your fingers, a pastry cutter, or a fork, work the butter into the flour until the mixture resembles coarse, pea-sized crumbs.
Pour in the chilled buttermilk and gently stir with a wooden spoon just until a wet, shaggy dough starts to form. Don't worry if there is still a bit of dry flour lingering at the bottom of the bowl, do not overmix!
Dust your work surface generously with flour and scrape out the dough, along with any remaining dry flour from the bottom of the bowl. Lightly flour your hands and gently pat the dough out into an even rectangle about ½-inch thick. (The patting and folding process will naturally incorporate all the remaining flour).
Using a bench scraper or floured hands, fold one-third of the dough over into the center, then fold the opposite third over top (like folding a letter). Gently pat the dough back down into a rectangle. Repeat this trifold process 2 to 3 more times to build distinct, flaky layers.
Pat the dough out to a final thickness of 1 inch. Flour a 2 or 2.5-inch round biscuit cutter and press straight down into the dough without twisting. Place the cut biscuits into your prepared cast-iron skillet so their edges are lightly touching. (Gently press remaining dough scraps together to cut out final biscuits).
Bake at 450°F for 15 minutes, or until tall, fluffy, and golden brown on top. Brush immediately with melted unsalted butter upon removing from the oven. Enjoy!