Preheat oven to 425. Sift flour, baking powder, sugar and salt into a large mixing bowl. Transfer to a food processor. Cut butter into cubes and add to flour, then pulse 6 or 7 times until the mixture resembles rough crumbs. (Alternatively, cut butter into flour in the mixing bowl using a grater, fork or a pastry cutter.) Return dough to bowl, add buttermilk and stir with a fork until it forms a rough ball (don’t worry if it is a little sticky).