Jason's Birthday Gift
Batman isn't completely against killing. He just feels like he can't be the one to make that decision, because "who gave him the right to be Judge, Jury, and Executioner?"
But, Jason's birthday is coming up and he thinks he has the perfect Idea for him.
Jason was resurrected. Apparently murdered Ghosts have a supernatural need to be avenged. He put 2 and 2 together.
He summons the Ghost King and asks if it's legal to Kill the Joker.
Cut to Jason'a birthday and everybody is having a blast. They get to opening the presents and finally get to Bruce's gift.
Jason opens it and finds a Signed Warrant for the Jokers Death, from the king of the dead Phantom.
He looks up at Bruce disbelieving, but he just nods back and continues drinking his coffee.
Jason: Did you just get me permission to Kill the Joker from fucking GOD!?
Bruce: Go break his Kneecaps kids
Alfred: I'll pack a lunch!
Bruce was staring at the wall of his office in the manor for the past three hours. Why you might ask? It was because Jason's second after-revival-and-return-of-proper-mind birthday waa two weeks from now and he couldn't come up with anything his second son would like.
While Bruce was able to come up with many ideas not even one of them was worthy of being even in the 'Good enough' category. Only one thing, Bruce knew, would be a perfect gift. The problem however was that Bruce, or rather Batman, didn't have the legal power to Kill the Joker (That and if Bruce were to have blood on his hands due to killing without proper permission he was scared that he won't be able to stop himself from doing so again). He isn't a judge, jury or an executioner. He is a vigilante doing his best to keep Gotham and the world as a whole safe.
Then Bruce realised that if he were to find someone willing to give him the legal power to kill the Joker he would take full advantage of it. So he started thinking of all of the world leaders who could give him that power. None of them unfortunately would agree to do so. For Bruce that just meant that he'd to find some other legal system to help him.
And the Infinite Realms had such a legal system. As the strongest land of the dead they had the authority to call for the head of the Joker.
Which leads us to Bruce one week before Jason's birthday kicking everyone out of the manor saying that he has some business with Constantine... Business he does indeed has... Business concerning the birthday gift he wants to give to Jason.
Bruce with the help of Constantine was able to summon Ghost King Danny Phantom and after an appropriate explanation he asked if it was possible for a death warrant to be issued.
It was naturally possible. And easy to do too. He had all of the needed paperwork due to many ghosts crying out for the Joker's death. He just needed to fill out the form and reference the paperwork and the magic of the Infinite Realms would create the warrant, making it so that even a nuke wouldn't be capable of sighing the parchment.
After the death warrant was ready it was beyond easy for Bruce to wrap it in the paper he uses for Jason's gifts.
Danny pops in a week later to check on the situation.
Danny: So! How'd he take it?
Bruce: He hugged me!
Danny: So the news that his ghost has been sent to the nightmare dimension forever?
Bruce: Would be good news yes.
Danny: Good! I'll leave you to it then!
Bruce: Would you like to stay for dinner?
*Thinking internally: Would you like to stay forever)
Danny: Oh! Yeah that would be nice.