Rihanna's Belated B-Day -- Booze & Cupcakes

Rihanna had a damn good reason to hit the Hollywood club scene last night -- she was finally attending her long-delayed 21st birthday party ... complete with loads of booze, a couple of decoys and NO Chris Brown in sight.

Sources tell TMZ the whole thing went down in a private patio at Coco de Ville. We're told a five-person party ordered everything from Grey Goose to Jack Daniels, a round of "fun" shots, some champagne, and a cupcake tower for the birthday girl.

TMZ has also learned Rihanna had planned to use a couple of decoys to keep her exit a secret, but TMZ cameras managed to get a shot anyway.

RiRi turned 21 on February 20 -- 12 days after the brutal fight with Brown. Relationships are complicated.