I used to always be really irritated whenever a recipe called for buttermilk, because it was always between one drop and half a cup, and no one wants to buy an entire quart because it will go bad. And somehow, buttermilk that has gone bad is even scarier than regular milk that has gone bad (which in itself is a true test of character).
Then I read this, which says that the buttermilk you buy in the supermarket isn’t even real buttermilk. (Actually, just now while searching for that article I found this, which says that that other article is a lie, but whatever. Slate is so angry.) So then I felt less bad about making my own “buttermilk” as a substitute.
It is super easy: Combine one cup of milk with one tablespoon of white vinegar or lemon juice. (I prefer lemon juice because I can’t stand the smell of vinegar, but my entire bottle of lemon juice recently leaked all over my fridge because I laid it on its side.)
EDIT: The slightly more correct way to do this is to add the tablespoon of acid first, then add milk up to the one-cup mark—so you’re adding very slightly less than one cup of milk.
Then wait a while—five or ten minutes or so. That’s it. It will look curdled and may be a bit clumpy. (At least, mine was, so I hope that is normal.) Don’t drink it, that would probably be gross. Just put it in pancakes or whatnot.