Using coarse holes of a box grater or a food processor with coarse shredding disc, shred butter. Transfer to a baking sheet lined with parchment paper as you go. “Shredding it means you end up with very uniform shards, which results in much flakier biscuits,” Ruzicka explains. Refreeze shredded butter 1 to 3 hours to be sure it’s crisp and totally frozen. (Also refreeze any clumps left on food processor, then shred by hand.)
Meanwhile, in a large bowl, combine 2 3/4 cups pastry flour, the whole-wheat and all-purpose flours, baking powder, and salt with your hands. Add butter; toss with your fingers to distribute, breaking up any large lumps. Gradually add 1 1/2 cups plus 2 tbsp. buttermilk and mix together with your hands, tossing and squeezing until crumbs hold together without being sticky. Work fairly quickly so butter stays cold. Add remaining 2 tbsp. buttermilk only if there are still dry patches. Gather dough into a cohesive but slightly shaggy ball.
Lightly flour a work surface with pastry flour. Set dough on top and sprinkle with flour. With a floured rolling pin, pound dough to flatten somewhat, then roll into a 9- by 12-in. rectangle about 1 in. thick. Using a bench scraper if needed to help lift dough, fold it in thirds like a letter, starting from a short end (don’t worry if it breaks apart a bit). “This creates layers, like a rough puff [pastry], except it’s biscuit dough.” Reflour work surface and rolling pin. Turn dough 90° and repeat folding process 2 more times. Tap sides with bench scraper to square off. Fold and roll 1 more time.
Using a floured chef’s knife, score dough in thirds lengthwise, then in fourths crosswise to make 12 squares; cut straight down. Then cut straight along outside edges to neatly trim. Set squares on the lined baking sheet and freeze until very hard, 1 to 1 1/2 hours. “To get the best rise and ensure a tender texture, the butter needs to be extremely cold. If it’s too warm, it will begin to melt out early in the bake instead.”
Meanwhile, preheat oven to 375°. Set biscuits on 2 (room temperature) rimmed baking sheets lined with parchment and bake 15 minutes. Swap pan positions and rotate pans 180°. Continue to bake until biscuits are deep golden and no longer wet in the middle (go ahead and break one open to check), 15 to 20 minutes more. “They go from looking like little hockey pucks, to rising, then voop!” Serve them warm as is.
MAKE AHEAD: Through step 4, frozen airtight up to 1 month. The baked biscuits, up to 2 days; reheat in oven before serving.