Make the dough: 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 (250g) 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 rosemary, kosher salt, baking powder, black pepper, baking soda, and remaining 2½ cups (313g) 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 flour/yeast mixture from medium bowl. 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).
Prepare topping: In another small bowl, whisk together sea salt, remaining 1 teaspoon (1g) 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.