3½ cups all-purpose flour (seems like a lot, it's correct)
1 cup (2 sticks) chilled unsalted butter, cut into ½-inch pieces (plus a tablespoon melted)
1 cup chilled buttermilk
Preheat oven to 425°. In a food processor, pulse baking powder, salt, sugar, baking soda, and 3½ cups flour. Add butter and continue pulse until largest pieces of butter are the pea-sized.
Transfer to a large bowl and gradually drizzle buttermilk over top, mixing with a fork as you go to incorporate. Knead mixture a few times in bowl until a shaggy dough forms (mixture will look a little dry), then turn out onto a clean surface and pat into a 1"-thick square. Don't make it perfect.
Using a knife or bench scraper, cut dough into 4 quarters. Stack each piece on top of one another, dust surface with flour and press down dough to flatten, incorporating any loose dry bits of dough as you combine.
Roll dough into a 1"-thick rectangle and, if you want, trim a thin border around sides of dough to create clean edges. (I just pressed them in so they were neater but not perfect.) Cut into a 4x3 grid to make 12 biscuits. Transfer to a parchment-lined baking sheet, spacing apart and freeze 10 minutes.
Brush tops of biscuits with melted butter and reduce oven temperature to 400°. Bake biscuits until deep golden brown on bottom and slightly golden on top, 20–25 minutes.