LITTLE FLOWERS OF SAINT FRANCIS OF ASSISI, Chapter 33
HOW ST CLARE, BY ORDER OF THE POPE, BLESSED THE BREAD WHICH WAS ON THE TABLE, AND HOW ON EACH LOAF APPEARED THE SIGN OF THE HOLY CROSS
An interesting comparison comes to mind between this story and the story of the budding of Aaron's rod. In the case of Aaron's rod, the miracle took place to confirm authority which had been challenged. In St. Clare's case, the miracle took place when she acted in obedience to the Pope's command, given to overcome her reluctance to assume what would usually be the Pope's prerogative.
Also interestingly, both stories mention preserving proof of the miracle, Aaron's rod or one or more of the loaves. In connection with this sort of confirmation miracle, I remember a comment from the burning bush. The confirmation that God sent Moses would be, he would bring the Isrealites to Mt. Horeb - or Mt. Sinai - and worship God there. Not a confirmation Moses could use to convince the Isrealites to follow him, but a confirmation to Moses that he had really been given a mission from God.