Last week I needed some egg whites for a cake and decided to buy the large carton thinking that I would use them in the allotted time of 7 days. I made SMBC to go on my cake , used some throughout the week for breakfast but still had lots left. Yesterday I decided that I would make a white cake today just to use them up, well I made the Party Cake from Dorie Greenspan's book and some Strawberry SMBC(no preserves, just extract) and still had plenty left. Well that meant breakfast for lunch.
Moral of the story is don't purchase the large carton unless you're sure you will need that much
I didn't have buttermilk, but had yogurt that needed to be used so I used that and topped it off with regular milk.