Buttermilk Biscuits

1 1/2 cups all-purpose flour
1 1/2 tablespoons sugar
1 1/2 teaspoons baking powder
Rounded 1/4 teaspoon salt
1/4 teaspoon baking soda
1/2 stick (1/4 cup) cold unsalted butter, cut into 1/2-inch cubes
3/4 cup well-shaken buttermilk
1 tablespoon milk or cream for brushing biscuits

Put oven rack in middle position and preheat oven to 425°F.
Sift together flour, sugar, baking powder, salt, and baking soda onto a sheet of wax paper, then sift again into a bowl. Blend in butter with your fingertips until mixture resembles coarse meal. Add buttermilk and stir with a fork until a dough just forms (dough will be moist).
Turn dough out onto a well-floured surface and knead gently 6 times. Pat out dough on a floured surface with floured hands, reflouring surface if necessary, into an 8- by 5 1/2-inch rectangle. Trim all 4 sides with a knife, dusting knife edge with flour before each cut. Cut rectangle in half lengthwise, then into thirds crosswise to form 6 (2 1/2-inch) squares, flouring knife between cuts. Transfer biscuits with a metal spatula to an ungreased baking sheet, arranging them 2 inches apart, and brush tops with milk or cream. Bake until pale golden, 12 to 15 minutes, then transfer to a rack to cool to room temperature.