You can make an edible bathtub-shaped cake topper from gum paste using a few common kitchen items. Gum paste is made by combining natural thickening gums derived from tree sap with confectioner's sugar. The resulting mixture is a pliable dough that cake decorators often choose for their most demanding topper forms. Finished shapes set up and permanently harden as moisture leaves the gum paste. A completed gum paste bathtub cake topper is instantly recognisable when all of the gum paste accessories are in place.
Dust a layer of cornstarch on a clean glass, marble, granite, stainless steel or Formica surface. Coat the rolling pin in cornstarch.
Tear off a golf ball sized chunk of gum paste. Knead the ball by hand until it becomes warm and pliable. Roll the ball of gum paste to a uniform thickness of 1/2 inch.
Flip and turn the gum paste sheet to a new position on the cornstarch covered surface. Dust the sheet with more cornstarch. Continue rolling, flipping and turning until the gum paste sheet is oval shaped and an even 1/8 inch thick throughout.
Remove the lid from the covered butter dish. Apply a thin layer of vegetable oil to all of the outer surfaces of the lid with a soaked paper towel. Drape the thin sheet of gum paste over the oiled lid.
Press the gum paste to conform to the shape of the lid. Cut a small hole and form the gum paste around the lid handle, if one is present. Trim the excess gum paste from around the base of the lid.
Form bathtub accessories like faucets, drain plugs, rubber duckies and shampoo bottles from the trimmed gum paste. Coat your fingers with cornstarch to prevent sticking. Use the scale of the bathtub form as a guide to create relatively sized pieces.
Dry all of the gum paste forms for 72 hours in a cool, dry and well-ventilated area. Paint the dried gum paste bathtub accessories with food colouring gels. Lift the dried bathtub-shaped cake topper from the butter dish lid.
Glue the colourful accessories into the bathtub with pasteurised egg whites. Hold each piece in position for two minutes to allow the egg white adhesive to start setting up. Dry the completed bathtub overnight to set up the edible glue completely.
Add the topper to the cake no more than 10 minutes prior to service. The fats in many icings cause the gum paste to deteriorate quickly.