In a large bowl (do not use metal), sift the sugar and make a well in the center. In a small saucepan, add the water and sprinkle the gelatin on top to soften for about 5 minutes. Begin to heat the gelatin and stir until the gelatin is dissolved and clear. Do not boil. Turn off the heat and add the glucose and glycerine, stirring until well blended. Add the flavoring. Pour into the well of sugar, and mix until all of the sugar is blended. Use hands to knead icing until it becomes stiff. Add small amounts of confectioner's sugar if the mixture is sticky.
Form the mixture into a ball and wrap tightly in plastic wrap. Place in an airtight container. This icing works best if allowed to rest at room temperature for about eight hours before using, particularly if the weather is humid. Do not refrigerate.
To cover a cake with fondant: Dust a clean pastry cloth, or a smooth, clean surface, with cornstarch and roll the fondant with a rolling pin until it is approximately 1/4 inch thick. Make sure that the fondant is large enough to fit over the top and sides of the cake. Slide both hands under the fondant and carefully center it on top of a cake that has been freshly iced with buttercream. (The icing makes the fondant adhere to the cake.)
Dust your hands with cornstarch and smooth the fondant, starting at the top and working down the sides until the entire surface is even and flat. Cut off the excess icing around the bottom of the cake with a pizza cutter or sharp knife. Decorate the cake with buttercream or royal icing. This fondant keeps a cake fresh for two days at room temperature. Do not refrigerate a cake with fondant icing.