The Chocolate Beam is an attack used by all forms of Majin Buu. This attack can change any object or person into a type of food or in one case in the anime series Ultimate Buu turns a rock into a toliet. Super Buu also used this attack to turn everyone on the Lookout into chocolate to eat them. This can also act as a way for Buu to absorb another, but only in the case of absorbing another Majin Buu, as with Evil Buu and the Fat one, possibly because the spell, or whatever it is, was used on another with same power so the victim retains their power without being able to use it as something of a container. When Baby Vegeta threw the change beam back at Majuub, Majuub didn't give Baby any extra strength, probably because Baby wasn't a form of Buu. It's mostly likely because he wasn't a version of Buu, because if it was because Majuub's own beam was knocked back at him, then Fat Buu wouldn't have kept any powers when Evil knocked his back.
Attack used to change things into certain objects or food the user may want