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Adjust oven rack to middle position and preheat the oven to 450°F (230ºC).
Using the large holes of a box grater, grate 1 stick frozen butter. In a medium bowl, toss together grated butter, flour, sugar, baking powder, baking soda, and salt until butter is thoroughly coated in flour mixture. Chill, uncovered, in the freezer for 10 minutes.
Add remaining butter to an 8-inch square aluminum baking pan. Place in preheated oven until melted, about 4 minutes.
Meanwhile: Using a flexible rubber spatula, stir buttermilk into the flour mixture until just combined (dough will be very sticky).
Remove pan from oven. Carefully scrape dough into the prepared pan, and, using the same spatula, spread dough to the edges. Use a butter knife to cut the dough all the way through into 9 equal squares. Sprinkle flaky sea salt on top.
Bake until golden brown and a toothpick inserted in the center comes out clean, 22 to 24 minutes. Let the biscuits cool slightly in the baking pan before serving, about 5 minutes.