Son Goku's obsession for confrontations and power is more extravagant than ever. Since Uub, Buu's reincarnation, wasn't available in Dragon Ball Z Kakarot, Goku had to find another way to train. The endgame sub story Pure Pair will give you the opportunity to fight Buu once again.
Pur Pair
In the end game, you'll first have to go to the entrance of the world championship stadium to talk to Baba who is just in front. Goku will want to find opponents that are worthy of him and will ask the witch to find some in realm of the dead. He'll have the idea of resurrecting Kid Buu to face him one last time. Once Baba accepts the request, go to the world of Kaio Shin, which can be displayed by pressing Y or triangle on the world map.
Kid Buu will be level 82, so prepare your team well. The fight won't be any different from the one that saved the universe, except for the setting and the cinematics, of course. Don't forget to prepare a few healing items, as Kid Buu can chain two Kameha in a row. When he prepares his charge, easily identifiable because a mini-cinematic is triggered, protecting you will be the most effective defense.
Unfortunately, the battle won't bring you any soul emblems, but a nice bundle of D-medals will be yours as soon as you defeat Buu.