In a small bowl, stir collectively heat buttermilk and yeast. Let stand till combination is foamy, 5 to 10 minutes.
In a medium bowl, whisk collectively 2 cups (250 grams) flour and sugar. Add yeast combination, and stir till simply mixed. Let stand for five minutes.
Line a rimmed baking sheet with parchment paper.
In a big bowl, whisk collectively 3 teaspoons (3 grams) rosemary, kosher salt, baking powder, 1½ teaspoons (4.5 grams) pepper, baking soda, and remaining 2½ cups (313 grams) flour. Add chilly butter, tossing to coat. Utilizing a pastry blender or 2 forks, reduce in chilly butter till items are smaller than peas and combination is crumbly. Stir in yeast combination. Utilizing your arms, knead dough in bowl till dough simply comes collectively.
Prove dough onto a flippantly floured floor; press or roll right into a 1-inch-thick disk. Fold dough in half; press or roll right into a 1-inch-thick disk. Utilizing a 2½-inch spherical cutter dipped in flour, reduce dough with out twisting cutter, and place about 1½ inches aside on ready pan. Reroll scraps as soon as by pushing scraps collectively, folding in half, and urgent or rolling to 1-inch thickness. Discard remaining scraps. Cowl and let rise in a heat, draft-free place (75°F/24°C) till puffed, about quarter-hour.
Preheat oven to 425°F (220°C).
In one other small bowl, whisk collectively sea salt, remaining 1 teaspoon (1 gram) rosemary, and remaining 1 teaspoon (3 grams) pepper. In a 3rd small bowl, whisk collectively egg and 1 tablespoon (15 grams) water. Brush egg wash onto biscuits; sprinkle with sea salt combination.
Bake till golden brown, 10 to 14 minutes. Let cool on pan for five minutes. Serve heat.
