Place an oven rack on or near the center shelf and preheat the oven to 425 degrees F. Line a baking sheet with parchment paper.
In a large bowl, whisk together the flour, kosher salt, baking powder, and sugar. With a pastry blender, a large fork, or two knives, cut the butter into the dry ingredients until the mixture looks like bread crumbs, with a few larger pieces here and there. (alternately, you can grate frozen butter. After grating, chill the butter for 10 minutes before using).
Pour in the buttermilk, then use a spatula to quickly and gently mix the dough approximately 15 times until just barely combined.
Pat the dough into a rectangle about 3/4" thick. Fold it into thirds like a letter, turn, pat down gently, and repeat the process 1-2 times. Roll gently with a floured rolling pin until the dough is 3/4" thick again, working the dough as little as possible.
Cut into circles with a biscuit cutter for traditional round biscuits. Do not twist the cutter, press straight down and up. Or, to avoid leftover dough scraps, cut the dough into squares or diamonds with a bench scraper. Place biscuits on the prepared baking sheet.
Optionally, you can brush the tops of the biscuits with buttermilk to enhance browning.
Bake for 15 to 20 minutes, until they're lightly browned on top. Remove from the oven, and serve warm (brushed with melted butter, if you're so inclined).
Wrap leftover biscuits and store at room temperature for several days. Freeze for up to 3 months. Re-warm before serving.