Mix dry ingredients together. Cut butter into flour mixture using a pastry knife or a food processor until crumbly. Add cranberries and mix gently to distribute.
Pour buttermilk over flour mixture and stir with a fork or pulse in food processor until the dough gathers into a sticky ball. Do not over mix or the scones will be tough.
On a floured surface, gently pat dough to 1/2-inch thick. Cut into 3-inch rounds using a floured cutter. Reshape leftover scraps with your fingers and cut more rounds. Place rounds 2 inches apart on an ungreased baking sheet. Brush with beaten egg.