My friend is a dental assistant and wanted me to make a tooth cake for her boss, the dentist, for his birthday.
I made a double layer sheet cake (one chocolate layer and one vanilla layer). I then took a large serrated knife and began to shape it into a molar. At first it seems as you are destroying the cake, but as you continue to work on it, it starts to take the shape of a tooth.
I then made a double batch of marshmallow fondant and rolled it out to cover the cake, which had already been frosted with regular icing. Do not skip this step or the fondant will not stick to the cake. I began to smooth it over the entire cake really pressing down into the groves I had cut in the cake to give it the appearance of a 3D tooth.
I then colored a small portion of fondant turquoise and shaped it into a large long rectangle to make a toothbrush handle. I then made a 3D rectangle out of white fondant and then used the prongs of a fork to create the bristle lines of the brush.