I'm hoping this question doesn't have a really easy answer that I am inadvertently overlooking. When I first wondered when Voldemort had originally learned of the existence of the Elder Wand, I picked through the chapter The Wandmaker in Deathly Hallows and re-read The Tale of the Three Brothers in Tales of Beedle the Bard, looking to see if there was any reference to exactly when Voldemort became aware of the existence of the Elder Wand. I've also checked out Pottermore's information on wands to see if J.K. Rowling sneaked any information regarding Voldemort and the Elder Wand into her segments on wandlore and wandmaking, but I didn't find anything.
In The Wandmaker, Mr. Ollivander says, after Voldemort fails to kill Harry with Lucius Malfoy's wand and Harry's wand splits Lucius's wand and destroys it, that Voldemort "went back" and began looking for the Elder Wand "again".
When did Voldemort first become aware of the existence of the Elder Wand?