I used 2 8" cake pans of one cake mix and a pyrex bowl with an 8" diameter opening for another full cake mix. I cut them were they bubbled up, so they sat flat on top one another, the bowl cake on top and upside down. Then I cut off used restaurant chopsticks and put two on each side of Barbie to anchor the cakes together. Then I cut out the middle hole with a knife and had to lift the cakes and have my niece shove the middle hole pieces down through the bottom.
I did not want to break any of your Barbies, so I thought 2 8" pans would be enough, but it wasn't, so I tossed her. Not wanting to spend the time to bake some more, I searched in your Barbie box for the Barbie that had legs jointed so much that she could bend her knees to 90 degree angles (In the picture where I am pulling her out, this is the configuration I had to put her in to fit). So she kind of squatted in your cake, she looks like a midget, but she was in there unbroken! Then I did a lame frosting job with a whipped frosting that didn't succumb to the paper smoothing treatment that the video shows works on buttercream frosting, and she waited in the refrigerator over night.