Preheat the oven to 375F.
Grate the frozen butter on a microplane, then freeze until ready.
Put flour in a large bowl with baking powder and salt, mixing with a fork, then add in the frozen butter and stir gently.
Add in half of the buttermilk and stir with the fork before adding in the rest of the buttermilk - this keeps the dough from becoming too wet - until the dough comes together.
Sprinkle a little flour onto a board or the counter, and dump out the dough. Using a bench scraper and a rolling pin, shape the dough into a thick rectangle and roll it out into a rectangle. Fold the dough onto itself, turn it, and roll again into another rectangle. Repeat a few times until the dough is soft and smooth.
Cut the final rectangle into 8 pieces, brush it with an egg wash, then place on a sheet pan and bake for 35 minutes until browned and crispy.
