In a bowl, combine the flour, corn starch, baking powder and salt. Use a pastry cutter to cut in the butter until crumbly. (If you want to add grated cheese and/or herbs, add those now.)
Stir in the milk just until moistened and use your hands to gather the mixture up into a craggy dough, but don’t overwork the dough. There will be some moist bits, and some dry bits, and that's exactly how it should be. If it seems too dry, add a little more milk, a tablespoon at a time. You don't want the dough too wet, though; it should be just moist enough to barely hold together.
On a lightly floured surface, gently roll the dough out to 1 inch thick, using a little more flour as needed to keep the dough from sticking.
Cut the dough into thirds with a bench scraper, stack the three sections, and roll the dough out again to about 1 inch thick.
Cut into rounds with a 2 1/2 inch biscuit cutter, trying to cut as many biscuits as you can the first time. Do not twist the cutter, just cut straight down and lift it straight back up. Gather up the scraps, press them together and cut those as well (the scrap biscuits don’t bake quite as prettily, but are just as tasty). You should be able to cut around 12 biscuits.If you like, you can also use a sharp knife or a bench scraper to cut square biscuits, which will reduce the amount of dough scraps that you need to re-roll. Line a baking sheet with parchment paper, and place the biscuits close together, about 1/2-1 inch apart. Freeze the cut biscuits for 30 minutes.
While the biscuits are freezing, preheat the oven to 450 F and position a rack in the center of the oven.
In a small bowl, beat the egg and water together with a fork to make an egg wash, then use a pastry brush to lightly brush the beaten egg over the tops of the biscuits.
Bake the biscuits until risen, fluffy and golden brown on top, about 15 minutes.
Cool biscuits for 5 minutes, then serve hot with softened butter, jam or honey.