The Recipe for Gertrude (ガートルードのレシピ, Gātorūdo no Reshipi) is a 5-volume serialization that ran in LaLa starting in 2001.
The North American serialization is available through CMX Manga (DC Comics).
From CMX Manga: Sahara is a normal high-school girl whose life is turned upside down when she meets "Gertrude," a 100-year-old demon who looks like a teenaged boy.
Gertrude searches for the recipe in order to learn more about his origin, and wants to destroy the formula so that it can never be repeated.
Sahara becomes a resourceful ally in Gertrude's quest, and they are aided by some very comical and not-very-threatening demons.