How to replace buttermilk with 1 tablespoon of vinegar or lemon juice mixed in 1 cup of milk. Sour cream diluted with milk or water to the consistency of heavy cream, natural unsweetened kefir, yogurt diluted with milk or water to the consistency of heavy cream. This buttermilk substitute is especially good for marinades (such as in a buttermilk brine for fried chicken), but it also works well for baking. First, consider having a box of buttermilk powder in your pantry so that you always have buttermilk ready and you'll never have this problem again.
But if you're working with a recipe that requires a small amount of buttermilk (for example, less than a cup) and you don't want to run to the supermarket, or if you're cooking and suddenly realize that you don't have anything, there are plenty of ways to hack homemade buttermilk. Although buttermilk was traditionally a byproduct of making butter, modern buttermilk is made by adding lactic acid bacteria to milk, which ferments it.