"I'm definitely doing her wedding dress," fashion's Stella McCartney, 36, told PEOPLE at a Tuesday night cocktail party she hosted at her West Hollywood boutique – where she arrived fashionably late along with the couple Charlize Theron and Stuart Townsend.
There's only one hitch before Johansson, 23, gets hitched to fiancé Ryan Reynolds (who proposed recently), according to McCartney: "She doesn't know it yet."
As for what the British designer, who created Madonna's gown for her 2000 wedding to Guy Ritchie, has in mind for the young bride-to-be, "You know, some kind of dodgy, black thing," McCartney, tongue firmly planted in cheek, joked. "Latex! Latex for Scarlett!"
On a more serious note, McCartney said, "I'm so thrilled for her."