Flaky Buttermilk Biscuits from Carla Hall

Name: Flaky Buttermilk Biscuits from Carla Hall
Ingredients: 8 tablespoons (4 ounces) unsalted butter, frozen, plus more, for the pan; 2 1/2 cups (310g) all-purpose flour, plus more for shaping the dough (if measuring by cups, be sure to stir the flour in the container first, scoop into the cup, then level off, without tapping the flour down); 1 tablespoon baking powder; 1 teaspoon granulated sugar; 1 teaspoon salt; 1/2 teaspoon baking soda; 2 tablespoons vegetable shortening, at room temperature; 1 1/2 cups whole-milk buttermilk, cold