- Preheat the oven to 425 F. Lightly grease a baking sheet or line with parchment paper. (Or, lightly flour a large pizza stone - my preference).
- Sift together the flour, baking powder, baking soda, and salt in a large bowl.
- Add the butter to the flour mixture. Using a pastry cutter, two knives, or a stand mixer, cut the butter into the flour mixture until it resembles coarse meal. Add the cheese, scallions, and parsley and stir until just combined.
- Add the buttermilk and mix lightly until the dough just begins to stick together, being careful not to overmix. Add up to 4 tbsp. more buttermilk, one at a time, if the dough is still crumbly and not sticking together.
- Turn the dough onto a lightly floured surface or piece of parchment paper. Knead several times until the dough just comes together. Form the dough into a flat round. Pat or roll to 1/2 to 3/4 inch thick.
- Using a floured biscuit or cookie cutter (or a wide mouth mason jar, lightly dusted with flour), cut the dough into rounds about 2 1/2 inches, pressing down without turning the cutter.
- Place the biscuits onto the baking sheet or pizza stone. Mix egg and milk to make the egg wash and lightly brush the tops of the biscuits.
- Bake until golden brown, about 12 to 15 minutes. Serve warm.