Line a ¼ sheet pan with parchment paper.
In a mixing bowl, combine 5 cups (624g) all-purpose flour, 2 tablespoons baking powder, 2 teaspoons salt and 4 teaspoons sugar.
Grate 24 tablespoons butter on a large hole grater.
Add shredded butter to flour mixture and lightly mix with fingers until combined and butter pieces are completely coated in flour.
Form a well in center of flour butter-mixture; pour 1¾ cups (408g) buttermilk into well. Using a spoon, mix dough gently until a shaggy dough comes together.
Transfer dough to a work surface and lightly press dough into a rough/shaggy 8✕12-inch rectangle (about 1-inch thick)
Fold rectangle into thirds (like a letter) and press down into a 1-inch thick rectangle. Repeat folding and pressing twice more.
Roll dough out between pieces of parchment paper into an 11 ½ x8-inch rectangle. Using a bench scraper, transfer to prepared sheet pan; press into pan to reach edges. Transfer to fridge and chill 1 hour.
Meanwhile, heat oven to 400ºF (204ºC) with racks set in middle and lowest positions.
Cut biscuit dough, in the pan, into 8 even pieces. Brush with buttermilk and sprinkle with 2 teaspoons flaky sea salt.
Transfer biscuits to middle rack and place a 1/2 sheet pan below on lowest rack to catch any butter drippings. Bake biscuits until tops are golden brown and crisp, 31–34 minutes. Remove from oven and let cool on sheet 5 minutes before transferring to a wire rack.