Either working with a box grater over a large bowl or using the large grating blade of a food processor, quickly grate the frozen sticks of butter and then freeze the butter for at least 30 minutes. Meanwhile, in a large bowl, whisk the flour with the baking powder, salt, sugar, and baking soda, and freeze the dry ingredients for the same amount of time as the butter.
Scrape the frozen butter into the dry ingredients and toss briefly to combine. Pour in the buttermilk and stir just until it forms a solid dough. Scrape the dough onto a lightly floured work surface and, using your hands, mold the dough into a 10-by-7-inch rectangle. Fold the rectangle in thirds like a letter and then rotate 90 degrees. Using a rolling pin, flatten the dough again into a 10-by-7-inch rectangle. Repeat the folding, turning, and rolling process 3 more times, ending with the dough shaped into a 12-by-10-inch rectangle about 1⁄2 inch thick. Trim the edges so you have a sharp, clean rectangle and then cut this rectangle into 12 equal squares. Space the biscuits at least 3 inches apart on 2 parchment paper-lined baking sheets and freeze for at least 2 hours or up to 3 days.
Heat the oven to 425°. Brush each biscuit with some melted butter and then sprinkle with sea salt. Bake 1 sheet of biscuits for 10 minutes. Then reduce the oven temperature to 400°, rotate the baking sheet, and bake the biscuits until puffed and golden brown, about 15 minutes. Transfer the biscuits to a rack and repeat to bake the second sheet of biscuits. Let the biscuits cool for 5 minutes before serving.