Heidi Montag got the best present a girl could ask for during her 21st birthday bash in Las Vegas on Saturday night. The Hills star used the soiree to perform her debut single, "Higher," and the crowd loved it, a source tells 24Sizzler.com. (The audience "cheered with excitement," says one guest.) Alas, Lauren Conrad wasn't there to share in the festivities, but we're sure she really wished she could be! (Yeah, right!)