In the bowl of a food processor, combine flour, baking powder, salt and butter. Pulse until no chunks of butter are visible. Add the buttermilk and beaten egg and process until the dough just comes together. Turn out onto a lightly floured board and knead until the dough feels smooth and springs back a bit. Put the dough in an airtight container at room temperature and let rest for at least 2 hours, up to overnight.