This is brilliant. It really doesn’t work as well when written though…

Joke Teller: Did you hear the Freudian knock-knock joke?
Joke Recipient: No.
Joke Teller: Knock knock.
Joke Recipient: Who’s there?
Joke Teller: I beat children.

Or, substitute the punchline with something equally dark. It’s my new favourite joke. Problem is, you’ve now already heard it.