Drop Biscuits with Cheddar and Scallions

Makes 10 biscuits 2 cups bleached all-purpose flour 2 teaspoons baking powder 1/4 teaspoon baking soda 3/4  teaspoon salt 1 cup grated extra-sharp cheddar cheese 1/4 cup thinly sliced scallion greens 1 stick (8 tablespoons) unsalted butter, frozen solid 1 cup cold buttermilk, a few extra teaspoons if necessary Adjust oven rack to lower-middle position...