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Blake and her Gossip Girl co-stars celebrate her 21st birthday with a roaring-20s themed party at the Palace Hotel in New York last week.

EVERYONE was there, including her real-life love, Penn Badgeley and former nemesis Michelle Trachtenberg... Well, not everyone, I guess. Where's little Jenny?

Well, it sounds like she missed one helluva a party, according to Life & Style, Blake even got the birthday song sung to her by none other than Michael Buble!

As for her romance with Penn? “They were very cozy,” said a party-goer. “They seem as if they’ve known each other forever.”

Well, when you're 21, one year can seem like forever!