What if I can't get the cake out of the jar in one piece?

If you are growing from cakes, it is best to piece.

If you are growing from cakes, it is best to remove the entire cake in one piece. This is why "tapered" mason jars (the types detailed on PF's page) are better than straight or shouldered jars. If you are casing, you can simply use a spoon to remove pieces of the substrate - this will serve as your crumbled substrate layer.