4 teaspoons baking powder
1/4 teaspoon baking soda
3/4 teaspoon salt
3 tablespoons chilled butter, cut into 1/2-inch pieces
1 tablespoon chilled shortening or butter, cut into 1/2-inch pieces
1 cup buttermilk, chilled
Preheat oven to 450°F. Line a baking sheet with parchment paper.
Combine Old Mill Plain Unbleached Flour, baking powder, baking soda, and salt in a large bowl. Using your fingertips, snap butter and shortening into dry ingredients pieces are no larger than small peas and mixture is crumbly. (The faster the better, you don’t want the fats to melt.) Make a well in center and pour in buttermilk. Stir just until the dough comes together; it will be very sticky.
Turn dough onto a floured surface. Dust top of dough with flour and gently fold dough over itself until a soft dough forms, about 5 or 6 times. Press into a 1-inch thick round. Using a 2-inch cutter, cut out biscuits by pushing straight down; do not twist cutter. Place biscuits on baking sheet, sides just touching. Gently gather scraps, press out 1-inch-thick, and cut out biscuits. (These scrap biscuits will not be quite as light as the first ones.)
Bake until biscuits are fluffy and golden brown, 15 to 20 minutes. Brush tops with melted butter, if desired. Makes 12 biscuits.