So I made it yesterday and was making some flowers with it. They turned out great. I decided to take a break, and started again this morning. My MMF is still a great consistency, but when I roll it, it cracks. Any advice?

You might try using a little crisco instead of corn starch to roll it out and see if that does the trick, or knead in a bit of crisco or glycerine.

I second that motion crisco helps!