Preheat oven to 425F/220C.
Sift the flour, baking powder, salt, and sugar into a large mixing bowl. Add the diced butter and rub it into the dry ingredients. Add the buttermilk and the egg and mix until a slightly sticky dough comes together. If the dough is too dry, add 1 Tablespoon at a time of the additional cream, milk, or buttermilk and mix.
Dump the dough onto a lightly floured countertop bring together with your hands. Be careful not to overwork the dough. Just bring it together and flatten down with a rolling pin or your hands so that the dough is about an inch tall.
Cut the dough with a round cutter that has been dipped in flour to prevent sticking. Cut straight down and then lift the cutter straight up. Pick up the cut scone, turn it upside down and put it on a baking sheet that is lined with parchment paper or a silpat mat.
Beat the egg and salt and bush on the top of the scone only. Try not to let the egg wash drip over the sides.
Bake in the middle of the oven for 10 minutes or until golden brown on top.
