The designer, wearing a sparkley gunmetal headband, handpainted three dresses -- while worn by model Anna Cleveland -- in the window of Bergdorf Goodman as part of Fashion's Night Out in NYC.
To appease the cheering crowd, Posen took a black marker, signed the window's backdrop "Zac was here" and then drew a smiley face on the gray T-shirt he was wearing.
"I'll take that!" shouted one of the more than 50 people gathered on Fifth Avenue, many of whom were eating cupcakes given out by girls from Glamour magazine.
Also in the crowd was Anna's mother, iconic Seventies model Pat Cleveland (good genes!), acting like any proud mom would -- by videotaping and photographing everything her daughter did (or withstood).
All in the name of fashion!