There are a number of substitutes for buttermilk in baking. For each cup of buttermilk, you can use 1 tablespoon of white vinegar or lemon juice plus enough milk to measure 1 cup. Stir, then let stand for 5 minutes. You can also use 1 cup of plain yogurt or 1-3/4 teaspoons cream of tartar plus 1 cup milk.