In this tutorial I'll show you how to make a Mad Hatter's Hat Cake Topper. It can be used as a decoration or a top tier. If you want the hat to have cake inside, simply substitute the polystyrene with cake.
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Use a piece of sandpaper to smooth the surface of the polystyrene. Make sure not to lose the bottom edge and the top edge. If you 'cut' into them you will lose the round shape.