I Googled for "Giant Killer Sundae" and didn't find much, except this blurb under a hybrid image of a Chicago neighbourhood:
"Now a Starbucks, this former site of Lockwood Castle, home of "The Giant Killer" the Chicago Area's biggest sundae. If you were able to eat it in one sitting, you were given a free one next time you visited. Served 4-6 people."
ETA: If you Google Image a "Giant Killer Sundae," you'll find LizzieMaine's avatar !