Rihanna's 20th Birthday!

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Rihanna, celebrating her 20th birthday with a bash – and cupcake fight! – at Les Deux in Hollywood. "There was frosting all over Rihanna's face and the group was hysterically laughing to the point of almost tears," a guest tells us. When partygoers (like Kanye West and Wilmer Valderrama) weren't flinging cupcakes, they offered a standing ovation after Chris Brown sang an a capella version of "Happy Birthday." While Brown has denied that he and the birthday gal are dating ("We're just friends ... we hang out," he told PEOPLE at a post Grammy party), a party guest tells us the two were cuddling and acting "very cozy all night."