Last week I tried to make cinnamon rolls and the recipe called for 1 1/2 cups of buttermilk, unfortunately I only had 1 cup in my house. Since I was in my pajamas and the weather wasn't exactly hospitable, I decided that there was no way I was going out to the store. However, I was already halfway through the recipe (I really thought I had everything before I started) and needed a solution quick!

So I poured through a bunch of food science and cooking tip books and was able to find a few good substitutions that ended up saving the day.

1 cup of buttermilk can be substituted with:

1 tbsp vinegar (or lemon juice) plus milk to equal 1 cup. Stir and let stand for 5 minutes.

or

1 3/4 tsp cream of tartar plus 1 cup of milk

I also learned that for baking you can use:
1/4 cup buttermilk powder plus 1 cup of water.
Remember to mix the powder with dry ingredients and water with wet.
I personally haven't tried buttermilk powder, but think it could be handy to keep on hand - once opened it will keep in the fridge for a year.