Cheddar-Scallion Biscuits

Preheat oven to 450°F. Line a large rimmed baking sheet with parchment paper.

Stir together all-purpose flour, sugar, baking powder, salt, and baking soda in a large bowl. Using your fingers, quickly toss and pinch butter into flour mixture until crumbly. Freeze 10 minutes to firm up butter.

Stir in buttermilk until a shaggy dough forms. Stir in Cheddar cheese and scallions. Turn out dough onto a lightly floured work surface. Pat dough into a rectangle, and cut into thirds. Stack the thirds on top of each other, and pat into a rectangle again. Repeat cutting and stacking procedure twice.

Roll dough into an 11- x 7-inch rectangle about 3/4 inch thick. Trim off edges to form a 10- x 6-inch rectangle, and cut into 12 (2 1/2- x 2-inch) rectangles. Arrange about 1 inch apart on prepared baking sheet. Reroll scraps once, if desired. Freeze until cold, about 10 minutes. Brush with egg, and sprinkle with more cheese and scallions.

Bake until golden brown, about 13 minutes. Let cool 5 minutes on baking sheet before serving.