What is ingredient a legendary sandwich?
Flavor enhancers.
Once you get 'pass' option when you try to feed Cici, talk to Tiffany at the Lab. Agree to help her go get Cici and choose either to help Cici (to get the Cici Sandwich ending and Tiffany tube gain sequence) or help Tiffany (to get the Tiffany Sandwich ending and allow continued use of the lab). After you make a decision, return to where Cici was to get the flavor enhancer ingredient.