I just made this cake this weekend. It really was so much fun and very entertaining to design the dress and dressing her up in it.
The barbie cake is chocolate chocolate chunk oced with fudge. The rose cake is strawberry cake with strawberry filling and iced with white fluffy vanilla buttercream.
The dress is all fondant. The white decorations on the white portion of the dress are white buttercream piped with a small round tip. The rose cake is covered with fondant, the ruffle is white buttercream and the roses are fondant. Check how to make fondant roses here.