Preparation Preheat oven to 375°F. Pulse baking powder, salt, sugar, baking soda, and 5 1/2 cups flour in a food processor. Add butter and pulse until the texture of coarse meal with a few pea-size pieces of butter remaining. Transfer mixture to a large bowl. Mix in buttermilk with a fork, then gently knead just until a shaggy dough comes together. Pat out dough on a lightly floured surface until 1 1/4" thick. Cut out biscuits with a 3" biscuit cutter, rerolling scraps once. Place biscuits on a parchment-lined baking sheet and brush tops with egg. Bake until golden brown, 30–35 minutes.
Preparation Preheat oven to 375°F. Pulse baking powder, salt, sugar, baking soda, and 5 1/2 cups flour in a food processor. Add butter and pulse until the texture of coarse meal with a few pea-size pieces of butter remaining. Transfer mixture to a large bowl. Mix in buttermilk with a fork, then gently knead just until a shaggy dough comes together. Pat out dough on a lightly floured surface until 1 1/4" thick. Cut out biscuits with a 3" biscuit cutter, rerolling scraps once. Place biscuits on a parchment-lined baking sheet and brush tops with egg. Bake until golden brown, 30–35 minutes.