In a small bowl, stir together warm buttermilk and yeast. Let stand until mixture is foamy, 5 to 10 minutes.
In a medium bowl, whisk together 2 cups (250 grams) flour and sugar. Add yeast mixture, and stir until just combined. Let stand for 5 minutes.
Line a rimmed baking sheet with parchment paper.
In a large bowl, whisk together 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 cold butter, tossing to coat. Using a pastry blender or 2 forks, cut in cold butter until pieces are smaller than peas and mixture is crumbly. Stir in yeast mixture. Using your hands, knead dough in bowl until dough just comes together.
Turn out dough onto a lightly floured surface; press or roll into a 1-inch-thick disk. Fold dough in half; press or roll into a 1-inch-thick disk. Using a 2½-inch round cutter dipped in flour, cut dough without twisting cutter, and place about 1½ inches apart on prepared pan. Reroll scraps once by pushing scraps together, folding in half, and pressing or rolling to 1-inch thickness. Discard remaining scraps. Cover and let rise in a warm, draft-free place (75°F/24°C) until puffed, about 15 minutes.
Preheat oven to 425°F (220°C).
In another small bowl, whisk together sea salt, remaining 1 teaspoon (1 gram) rosemary, and remaining 1 teaspoon (3 grams) pepper. In a third small bowl, whisk together egg and 1 tablespoon (15 grams) water. Brush egg wash onto biscuits; sprinkle with sea salt mixture.
Bake until golden brown, 10 to 14 minutes. Let cool on pan for 5 minutes. Serve warm.