Place gelatin and water in a small saucepan over low heat and melt it. Combine the remaining ingredients in the bowl of a stand mixer. Add the melted gelatin to the mixer and beat at low speed until combined. This will form a paste or dough. Place in the refrigerator for a few minutes to let harden slightly.
Use a rolling pin and roll the fondant to a thickness of about 1/4-inch thick. Use cornstarch to "flour" the work surface and the rolling pin. Roll the fondant around the rolling pin to transfer it to the top of the cake.
The next task will be easier if you use a turntable. Place the fondant over the cake and unroll it. Use your fingers to gently position the fondant and smooth the top and sides. Be careful to avoid folds. It may be easier if you gently stretch it into place. Use a paring knife and cut away the excess using the bottom of the cake as the guide. Reserve the rest for a future cake.
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