This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Buu the Bottomless Pit side quest.
Time frame: From Majin Buu Reborn - Episode 8: Finale to No time limit
Reward: 249,984 EXP, D Medal, Card 086
Where to start: Southeast Mountains Area, as Gohan.
To start this mission, you need to contact Mr. Satan. Buu is waiting for you - he has a new substory.
The first objective is to catch a big fish. Take the fish to the quest giver, and you will get the next objective.
Talk to your mother and get some ingredients. Their locations are marked on the map. You can start off hunting for a gold game. Its skin glows golden, so it is easy to spot.
Dinosaur meat is also easy to get. They move in this part of the map and there are several places where you can find them.
As for rice, you can either acquire it during exploration or buy it from a merchant. Take the ingredients to mom and prepare a meal for Buu. This will complete the mission.