 He handed over that cassette to Madonna, who went to the backroom and finished the melody and the lyrics of the song, while Leonard worked on the other parts of the song.After finishing the lyrics, Madonna declared that she wanted the song to be named "Who's That Girl" and changed the movie to the same, rather than "Slammer", considering it to be a better title.In Fred Bronson's "The Billboard Book of Number 1 Hits" book, Leonard explained that the song was recorded in one day with Madonna adding her vocals only once.