1cupcold unsalted butter (2 sticks)cut into 1/4-inch cubes
1 1/4 to 1 1/2cupscold buttermilk
Preheat oven to 425º F. Line a baking sheet with parchment paper and set aside.
Combine flour, baking powder, baking soda, and salt in a large bowl. Cut the butter into the mixture using a pastry blender or two knives until the mixture resembles coarse meal (Alternatively, pulse dry ingredients in a food processor bowl fitted with the metal blade. Add butter and pulse 10 to 12 times. Transfer to a large bowl before continuing.).
Add 1 1/4 cup of buttermilk and mix gently until the mixture begins to clump together, taking care not to over-mix. If the mixture seems too dry, add additional buttermilk a little bit at a time, until the loose bits stick together when pinched between your fingers.
Pressing gently with your hands, gather the dough together into a rough-looking ball and transfer it to a lightly floured surface. Pat or roll the dough to 3/4 to 1-inch thickness. Cut with a 2 1/2 to 3-inch round cutter, dipping in flour as needed to prevent sticking. Gather scraps and gently press dough together and flatten it to make more biscuits.
Place biscuits on prepared baking sheet and brush with egg beaten with the milk.
Bake until pale golden brown, about 12 to 15 minutes. Remove from oven and serve immediately.