Jon Kopaloff/FilmMagic; Matt Dunham/AP
11/29/2007 AT 02:35 PM EST
Pharrell Williams stepped out Wednesday night for the launch of his New York store featuring his Billionaire Boys Club clothing line and Ice Cream sneakers. But, turns out, the in-demand producer (who recently contributed the song "Why Should I Be Sad" to Britney Spears
) is working up a Material project of a different sort – a musical collaboration with Madonna. "It was sick," Williams told PEOPLE about working with her. "I don't want to say so much. I'd rather for you to hear. It's mean though." Uh, mean? "It makes the speaker bleed," he says, clarifying. "It's heat."