I just opened a carton of heavy whipping cream with a sell by date of March 6th to find it had a thick, slightly fluffy layer of.. something on top. It's definitely not spoiled and the cream underneath is totally usable. My question is, what formed on top? Did I accidentally make butter by neglecting the carton? Should I toss it or put it on a baked potato? I'm both confused and intrigued. Thanks in advance!