1. Adjust oven rack to middle position and heat oven to 475 degrees. Whisk flour, baking powder, baking soda, sugar, and salt in large bowl. Combine buttermilk and 8 tablespoons melted butter in medium bowl, stirring until butter forms small clumps (see photo below).
2. Add buttermilk mixture to dry ingredients and stir with rubber spatula until just incorporated and batter pulls away from sides of bowl. Using greased 1/4-cup dry measure, scoop level amount of batter and drop onto parchment-lined rimmed baking sheet (biscuits should measure about 2 1/4 inches in diameter and 1 1/4 inches high). Repeat with remaining batter, spacing biscuits about 1 1/2 inches apart. Bake until tops are golden brown and crisp, 12 to 14 minutes.
3. Brush biscuit tops with remaining 2 tablespoons melted butter. Transfer to wire rack and let cool 5 minutes before serving.