When a recipe calls buttermilk and you don't have any, here's what you can do: for every cup of buttermilk called you need, mix 1 tablespoon vinegar with 1 cup milk. Let the mixture sit for 10 minutes or so. The combination will come close to the tangy creaminess of buttermilk.)
What is buttermilk, anyway?
Real buttermilk is the sour liquid left over from the process of churning cream into butter. That's usually not what you get when you buy a carton at the store, however. Most commercial buttermilk is made by adding a culture to milk (like you would to make cheese). As the culture converts lactose to lactic acid, the milk thickens and sours. (Commercial sour cream is made by a similar process.