Prep: Preheat your oven to 425 F (220 C). Line a 18 by 13 inch baking sheet with parchment paper, and set aside.  

Make the dough: Whisk together the egg and milk in a large glass measuring cup, until completely combined. Set aside 3 tablespoons of the mixture (for glazing your scones). Add the sourdough discard into the egg mixture and stir until no traces of sourdough remain. Set mixture aside. 

In a large mixing bowl, whisk together flour, baking powder, salt and sugar. Rub the butter in by hand or with a pastry cutter until the mixture resembles fine breadcrumbs. 

Make a well in the center of the flour mixture, and pour in half of the milk mixture. With a large open hand gently mix the ingredients together until a shaggy dough forms. Add the remaining milk mixture and incorporate any leftover dry bits of flour. 

Knead: Turn the dough out onto a clean surface and fold the dough on itself 4 or 5 times or until no traces of dry flour remain. Use a light hand and don't overwork the dough, but make sure the dough is cohesive and well incorporated and not crumbly. 

Shape: Pat the dough down until it is an even 1-inch thickness. Using a 3-inch biscuit cutter, cut out your scones and transfer them to your lined baking sheet. Re-roll your dough and repeat. 

Bake: Glaze scones with reserved milk and egg mixture.  Bake scones for 10 minutes at 425 F (220 C) and then reduce oven temperature to 325 F (160 C) for another 10 minutes. Don’t over bake or your scones will turn out too tough and crusty. 

Enjoy and store: Take scones out of the oven and place on a wire rack to cool. Eat warm, filled with jam, butter or cream.