Preheat oven to 425°. Line a baking sheet with parchment paper.
In a large bowl, stir together flour, sugar, salt, baking powder, and baking soda. Using two forks or a pastry blender, cut in cold butter until mixture is crumbly. Stir in cold buttermilk until a shaggy dough forms.
Turn out dough onto a lightly floured surface. Pat dough into a rectangle, and cut into fourths. Stack each fourth on top of each other, and pat down into a rectangle again. Repeat process 3 more times. Pat or roll dough 1 inch thick. Using a 2½-inch round cutter dipped in flour, cut dough without twisting cutter, rerolling scraps as necessary. Place biscuits 2 inches apart on prepared pan. Freeze until cold, about 10 minutes. Brush with egg; sprinkle with sea salt.
Bake until golden brown, about 15 minutes.
Notes
*We used White Lily.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/buttermilk-biscuits/