Any way, it seems like I have a lot of recipes that call for buttermilk: my Grandmother's Red Velvet Cake, my go-to white cake, and my son's favorite banana cake to name a few. I have always known that there was a way to make a buttermilk substitute, but it always made me nervous to try since the buttermilk was a key ingredient in my mind. So I have always bought the buttermilk and watched half of the container go to waste because I ran out of time to make something else with the buttermilk. (I realize buttermilk is a form of sour milk so the "use by" date might be a little facetious, but I have always faithfully followed it. )
A couple of months ago, I found out at 4pm that we were going to a friend's house for dinner at 6:30pm and I was in charge of dessert. After looking through my pantry and fridge, I realized I had everything to make banana cake, even the ripe bananas, except for buttermilk; however, I did have milk and vinegar so I decided to try making a homemade buttermilk substitute AKA sour milk.
For each 1 cup of sour milk, put 1 Tablespoon of vinegar in a glass measuring cup. Then add enough milk to make 1 cup of total liquid. Stir and let stand for at least 5 minutes before using. Use the same amount of sour milk as you would buttermilk.
(Always measure liquid ingredients at eye level and in a measuring cup made for liquids.)
I was a little nervous in making the sour milk, but the cake got rave reviews and I have used the sour milk as a buttermilk substitute in every recipe calling for buttermilk since and I even used skim milk to make it.
So now my son just waits for our bananas to ripen or in his mind rot so we can make his favorite banana cake.