Preheat oven to 425°. Pulse baking powder, salt, sugar, baking soda, and add the two flours (3½ cups) in a food processor to combine. Add chilled butter and pulse until the largest pieces of butter are the size of the proverbial peas–I like to go a little smaller here than I would if I was adding the liquid in the food processor (when I choose garbanzo bean-sized) because it won't get broken down much further. Then transfer to a large bowl and gradually drizzle 1 cup buttermilk over top, tossing with a fork as you go to incorporate. Barely knead the mixture a few times in bowl until a shaggy dough forms (mixture will look dry). If you think it needs a bit more buttermilk, drizzle an additional tablespoon or two onto the mixture. You want to be careful not to add too much, as the flour will continue to hydrate and it will be considerably less dry when you begin patting and rolling it out. Then dump out onto a clean surface and pat into a 1"-thick square. Just be careful to be as gentle as possible.

Using a knife or bench scraper, cut dough into 4 equal pieces and stack the pieces on top of one another, sandwiching any loose dry bits of dough between layers, and press down to flatten. This is gonna create some amazing layers! Lift up the dough to make sure it isn't sticking and dust surface lightly with flour. Roll dough into a 1"-thick rectangle and trim a thin border around sides of dough to create clean edges. I hate instruction like this in baking because I hate food waste, but this is important. Those clean edges allow the biscuits to expand and get very very fluffy. Save your scraps and loosely moosh (technical term) together. This is your Frankenstein/lucky/cobbled biscuit and you can award it to whomever you please. Will it be slightly less tender? Yes. Will the recipient care? Not a chance. Cut the rectangle of dough into a 4×3 grid to make 12 biscuits. Transfer (along with Frankenstein) to a parchment-lined baking sheet, spacing 2" apart; freeze 10 minutes.

Brush tops of biscuits with melted butter and place in oven. Reduce oven temperature to 400° and bake biscuits until deep golden brown on bottom and golden on top, 20–25 minutes.

Bon Appetit says you can make these ahead, but we have never been disciplined enough to not want to eat them all now. BAs Do Ahead: Biscuits (unbaked) can be made 1 month ahead. Freeze, uncovered, on baking sheet until solid, then transfer to a resealable plastic bag. Do not thaw before baking, but add a few minutes to the baking time.