Place a rack in the middle of the oven and preheat to 425°F. Line a baking sheet with parchment paper. Trim the root ends from 8 scallions and thinly slice crosswise; set aside. Melt 2 Tbsp. butter and set aside. Whisk together 2½ tsp. salt, black pepper, baking powder, baking soda, sugar, and flour in a large bowl.
Cut 10 Tbsp. chilled butter into ½” pieces and add to dry ingredients, tossing to coat. Work the butter into the dry ingredients until there are pea-size bits. Add scallions and toss to distribute. Create a well in the center and add 1¼ cups sour cream. Mix with a fork until large shaggy clumps form.
Turn out onto a lightly floured surface and knead once or twice until the dough comes together. Pat dough into an 8×4” rectangle about 1” thick. Fold dough in thirds, then pat into another 8×4” rectangle. Fold in thirds again and pat into an 8×4” rectangle. Cut lengthwise, then cut each half crosswise into 4 squares (8 biscuits total).
Transfer biscuits to the baking sheet, brush tops with melted butter, and sprinkle with flaky sea salt. Bake until golden brown, 18–22 minutes. Serve warm with sour cream and more butter.