Set the oven to moderately hot (gas 5 or 190°C/375°F). Line a Pyrex bowl or dome-shaped cake tin with baking paper. You can watch our video on how to line a round cake tin if you need help.
Weigh the eggs and then use the same weight for each of the caster sugar, butter and flour.
Cream together the butter and sugar until light and fluffy and the mixture is pale in colour. Beat in the eggs, one at a time, adding a little flour with each egg. Fold in the remaining flour.
Pour the sponge mixture into the lined cake tin, and level the surface. Bake in the centre of the oven for 20-25mins, or until the sponge has risen, is a golden colour, and springs back when lightly pressed in the centre. Then leave to cool completely before icing.
When completely cool, cover the sponge with a thin layer of buttercream icing and then lay the white fondant icing over the top and make pleats in the front to look like a skirt.
Remove Barbie's legs and cut a hole in the top of the cake and slot her in.
Decorate Barbie's top with the coloured fondant icing and the top part of the iced sponge cake to look like an overskirt and top.
Add silver balls and other decorations as required to make the skirt look pretty.