Place 4 cups confectioners' sugar in a large bowl. Make a well in the center and using a wooden spoon, stir in the lukewarm gelatin mixture. Mix in sugar and add more a little at a time, until stickiness disappears. Knead in remaining sugar. Knead until the fondant is smooth, pliable and does not stick to your hands.
How to Flavor Rolled Fondant. Flavoring Fondant is a lovely touch. Stir in a few drops of your favorite flavoring, such as almond, lemon, rose water or orange extract in place of vanilla in the recipe. I found lots more tips on how to work with fondant on bhg.com.
Put the water in a small microwave-safe bowl. Sprinkle the gelatin powder evenly over the surface of the water. Whisk 1-2 times to combine. Let the gelatin "bloom" for 5 minutes. Heat the gelatin in the microwave for 15 seconds to melt. Whisk the corn syrup and glycerine into the warm gelatin (see note if you want to tint the fondant).
All you have to do is melt the marshmallows with a tad bit of water then stir in the powdered sugar. Eventually, the powdered sugar will make the mixture a bit too stiff for you to stir easily. That’s when you need to dump the mixture out on the counter and knead the rest of the sugar into the fondant.