Jane Krakowski put it ALL out there -- boobs and butt included -- for a lounge singer styled performance totally reminiscent of her "30 Rock" days. Jane was the showstopper of the night at a gala for the Roundabout Theatre in NYC on Monday ... and never missed a beat when her full ass crack and…

"30 Rock" funny ladies Tina Fey and Jane Krakowski shared camera time at the same event in NYC on Tuesday. Question is ...