Preheat oven to 425°F Line a baking sheet with parchment or a Silpat.
In a food processor, combine 1 1/2 cups flour, baking powder, sugar and salt. Process briefly to combine. Add ice-cold butter cubes and pulse five or six times, until the butter is cut into chunks no bigger than a small pea. Add raisins and cranberries and pulse once or twice more to distribute. Add milk or buttermilk and pulse until dough is just combined and shows no large spots of dry flour; add more flour if dough appears too wet.
Sprinkle half the remaining flour on a board. Turn out the dough onto the flour and sprinkle with the last of the flour. Knead by hand just 3 or 4 times. Pat into a disc about 1 1/4 inches thick (about 6" across). Using a long, sharp knife, cut disc into 6 wedges. Cut straight up-and-down, and do not drag the knife through the dough, or it will rise unevenly. (Alternatively, use a 2.5-inch round cookie cutter and cut into 6 rounds, or pat into a 7.5x5 inch rectangle and cut into 6 2.5-inch squares.).
Place scones on lines baking sheet. Beat egg yolk and milk, and brush onto each scone. Sprinkle each scone with a small pinch of coarse sugar. Bake in the middle of the preheated oven for 13-15 minutes or until golden brown. Cool 5 minutes on pan, then transfer to a wire rack until cool enough to handle. Serve warm with butter or clotted cream and jam.