Heat oven to 375 degrees and line a large sheet pan with parchment paper or foil.
Coarsely grate the butter onto a plate, then freeze until cold and hard, at least 10 minutes. Meanwhile, in a large bowl, whisk together the flour, sugar, baking powder and salt.
Add the butter to the dry ingredients. Using a spoon, toss together until all of the butter is coated with flour.
Add half the buttermilk and toss with the spoon. When incorporated, add the rest of the buttermilk and gently toss again, without mashing together or overmixing, until the dry ingredients are lightly hydrated throughout. The mixture will be crumbly.
Flour a clean surface and dump the mixture directly onto it. Using your hands, gently press the crumbs together and then use a floured rolling pin to roll the mass gently but firmly into a 1-inch-thick rectangle.
Fold the dough in half: Using a bench scraper, lift the top half off the surface and fold it over the bottom half. This step may be crumbly and messy at first, but just go for it and fold what you can down from the top. Repeat this roll-and-fold motion 5 times, flouring the surface and dough as needed and using the bench scraper to straighten the edges as
Build the final layer: Fold the dough in half one last time, then roll to about 1½ inches thick to create a 6-inch square, using the bench scraper to straighten out the edges.
Using the bench scraper or a sharp knife, cut straight down into
the square to create a 3-by-3 grid of 9 squares, then place them on your sheet pan, upside down if you’d like taller biscuits.
Bake for 25 to 35 minutes, rotating the pan halfway through, until risen, golden brown on top, and slightly pale on the sides.
Don’t worry if a couple of the biscuits tip over or if melted butter pools underneath. Brush the tops with melted butter and sprinkle with flaky sea salt, if using