I used a sheet cake pan for the base and decorated this Rubber Ducky cake with buttercream frosting that I dyed the desired colors. I made the frosting the day before to save time but then I still had to re-whip it. I suggest putting the frosting in the fridge when you are not using it. The frosting in the pipping bag can get warm from your hands and it makes it hard to decorate with, if you put the pipping bag in the freezer for a minute it gets stiff again.
I used a Wilton ducky cake mold and again used buttercream frosting to decorate it. For the bubbles, towel, soap and bow I used rolled fondant, I dyed some of it lilac to give the cake a softer look, not so boyish.
This was a fun cake and a big hit. It does take time so make sure you have plenty of it. I got started too late and finished just before guests started arriving and the cake was a bit rushed. My daughter just loved her ducky cake.