I don't bake very often, so when a recipe calls for buttermilk, I rarely have it in my fridge. However, I almost always have milk and lemons -- and that's all you need to make your own buttermilk. Here's a quick trick I use to avoid an unnecessary trip to the store.
Follow these 2 simple steps:
1. Combine 1 cup of milk and 1 tablespoon of lemon juice in a small bowl.
2. Let the mixture stand 5 minutes.
That's it -- it's that easy! Now your homemade buttermilk is ready to use in your recipe as directed.
Tip: This will also work with low-fat milks and distilled vinegar instead of lemon juice.