Preheat oven to 400ºF. In a food processor or a large bowl, mix 3 cups of the flour with the salt, sugar and baking powder. Cut the butter into the dry ingredients until the mixture is textured like cornmeal. Separately mix the egg with the milk, and stir all but 2 tbsp. of this liquid into the dry mixture. Mix just enough to make a uniformly moistened dough. Dust a clean work surface with 1/2 the remaining flour, and turn the dough out onto the floured surface. Sprinkle the top with the rest of the flour, and with floured hands, gently push the dough into a circle about 1/2" thick.  Cut with a 2" or 3" biscuit cutter, and place the cut biscuits close together on a greased corn meal coated cookie sheet. You will get about 12 biscuits. Brush the tops of the biscuits with the reserved milk egg mixture. Bake in the preheated oven until puffed and brown, about 15 minutes. Cool for a few minutes before serving. Yields about a dozen biscuits.