I honestly will hardly ever have buttermilk on hand in my fridge. I run into this a lot when I want to make a recipe but it calls for buttermilk. It is also crazy to me to have to buy a big thing of buttermilk when you only need like a 1/3 of a cup. Well my mom forever ago told me how to make my own buttermilk. I know longer have to go buy a big thing of it and waste it. You can make it from things you already have in your kitchen.
Pour as much milk as your recipe calls for. So say your recipe calls for 1 cup of buttermilk. Measure out one cup of buttermilk. For every cup of milk you will need 1 tablespoon of vinegar or lemon juice. Either one works great. So use whatever you have on hand. Let it sit for a 5-6 minutes. You will notice the milk starting to curdle a little bit. This means that it is done. There you have it you have made your very own buttermilk. Some times with lemon juice you have to let it sit a little bit longer. I would say 10 minutes for lemon juice, and 5-6 for the vinegar.