Buttermilk

you can easily make your own "buttermilk" but taking 1 cup of milk and adding a couple teaspoons of cider vinegar to it (lemon juice works too), stirring and letting it sit on the counter for 10-15 minutes. The milk will curdle a bit and might even separate but it's fine to use. Even milk that has soured from age is safe to use in baking recipes that call for buttermilk.