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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Put some pieces of paper towel underneath the paste to get a proper shape of the brim. Make sure you are happy with the shape before leaving it to dry. If the paste is a bit floppy come back to it from time to time to check if the brim didn't lose its shape.