The "Umbrella" singer - who doesn't turn 20 until next Wednesday (Feb 20, 2008) - was partying with friends including Kanye West and actor Wilmer Valderrama at Los Angeles' Les Deux nightclub when the food fight began on Tuesday night (Feb 12, 2008).
An onlooker said: "There was cupcake frosting all over Rihanna's face and the group was hysterically laughing to the point of almost tears!"
Chris Brown, who is rumored to be dating Rihanna, was later given a standing ovation after he sang her an acapella version of "Happy Birthday."
Despite Chris insisting he and Rihanna are "just friends," onlookers at the exclusive club said the pair were "cuddling and acting very cozy all night."
The couple also attended the Grammys together last weekend (Feb 10, 2008) and stocked up on goody bags.
Rihanna picked up Balenciaga sunglasses, while 18-year-old Chris took home two pairs of Marc Jacobs sunglasses in red and blue.
When they realized they might be photographed together, Chris insisted they should leave.
A Grammys source said: "Chris was telling her they needed to go, and in seconds they were gone."
