What happens if you're in the middle of a big baking project and suddenly realize you're out of buttermilk?.Here are five quick, homemade substitutes for buttermilk using ingredients you likely have on hand:.1 tablespoon white vinegar + enough milk to make 1 cup.1 tablespoon lemon juice + enough milk to make 1 cup.1 ¾ teaspoon cream of tartar + 1 cup milk.¾ cup yogurt + ¼ cup milk.