Which Sesame Street magician says 'A la peanut butter sandwiches'?

The Sesame Street magician who says “A la peanut butter sandwiches” is The Amazing Mumford.

The Amazing Mumford is a flamboyant, W. C. Fields-esque magician whose tricks frequently go awry. His signature incantation, “À la peanut butter sandwiches,” is the phrase he uses when attempting to make magic happen. The deliberately silly wording fits Sesame Street’s tradition of turning familiar performance conventions into playful language lessons.

Mumford’s sketches usually center on a simple magical goal that goes wrong in an amusing way. The joke is not that he lacks enthusiasm; it is that his confidence and theatrical presentation repeatedly collide with the unpredictable results of his spells. His appearances therefore combine wordplay, comedy, and gentle parody of stage magicians.

The other choices belong to different parts of Sesame Street’s character world. Guy Smiley is a game-show host, Sherlock Hemlock is a detective parody, and Professor Hastings is a lecturer character. None is the magician associated with the peanut-butter catchphrase.