6 tablespoons cold unsalted butter, cut into 1/2-inch dice
2 1/2 cups plus 2 tablespoons buttermilk
Preheat the oven to 450°. In a large bowl, whisk the flour with the baking powder, sugar, salt and baking soda. Using a pastry blender or 2 knives, cut in the shortening and the butter until the mixture resembles coarse meal. Make a well in the center and add the buttermilk. Using a wooden spoon, stir just until a dough forms. Turn the dough out onto a lightly floured surface and knead 3 or 4 times.
Pat or roll the dough out to a 1/2-inch-thick round. Using a lightly floured 2 1/2-inch round biscuit cutter, stamp out 24 biscuits as close together as possible. Transfer the biscuits to an ungreased baking sheet and bake for 15 to 20 minutes, or until the biscuits are risen and golden. Serve warm.
The biscuits can be baked early in the day and reheated in a 350° oven. Alternatively, the unbaked biscuits can be refrigerated overnight on the baking sheet or frozen for up to 1 month.