Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper.
In a large bowl, whisk together flour, sugar, kosher salt, baking powder, and baking soda. Using 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 procedure 3 more times. Pat or roll dough to 1-inch thickness. 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 wash, and sprinkle with sea salt.
Bake until golden brown, about 15 minutes. Serve warm.
Notes
White Lily is a finely milled soft winter wheat flour that is very popular throughout the Southern US but can be hard to find elsewhere in the country. Visit whitelily.com for a store locator to find this flour in your area or to order online.
Recipe by Bake from Scratch at https://bakefromscratch.com/brian-hart-hoffmans-buttermilk-biscuits/