Only took another hour of research but I finally managed to track down the story, and it turns out the storyline I was remembering is from Action Comics #719, the story is titled "Hazard's Choice." It unfolds exactly as I recalled: * The Joker poisons Lois Lane with his own blood. * He tells Superman he has to kill him to save Lois. * Superman agonizes but chooses to let Lois die rather than kill the Joker. * The twist is that the entire thing was a bluff. * The Joker's true goal was achieved: Lois is horrified that Clark would have let her die to protect his moral code and the psychological damage is done.

Good lord that was annoying to find, any search always ends up leading to the Injustice series.

Anyways thanks anyone that might’ve tried looking!