Aarrgghh

This is a major mistake (the correct phrase is “the best chocolate in the world”): “bestest” would be like saying “migliorissimo” in Italian. There are, however, mother-tongue English speakers who use the term “bestest” as a joke, but the fact that the owners of this shop have called their establishment “the bestest of the world” and not “the bestest in the world” would suggest that this is a case of a genuine grammatical error!