Ethan Miller/Getty Images for Cirque du Soleil
Although Holly attended Hef's annual blowout at the Mansion last week, she spent Halloween in Sin City with her new man at the gala opening of his new show, Believe.
While she left Sunday after spending the weekend in Sin City, it seems Hef's former best girl just can't get enough of Criss.
"She left to go back to L.A. but is coming back Tuesday to spend some more time with him," a source tells E! News. "You can definitely say they are official."
—Reporting by Whitney English