What to substitute for buttermilk: If you don’t keep buttermilk on hand, you have a couple of options. You can swap 1 cup nonfat or low-fat milk mixed with 1 tablespoon lemon juice or vinegar for each cup of buttermilk. Or keep a can of nonfat buttermilk powder in your refrigerator—look for it in the baking aisle. Mix according to package directions. Plain yogurt also will work as a buttermilk substitute.