In a medium to large size saucepan, add your sugar, Jell-O packet, and water. Mix until no lumps remain, then place over medium heat and bring to a boil, stirring occasionally.
When your mixture boils, turn your burner temperature down too low, and let your mixture simmer on low for 10 minutes without stirring. Your mixture will expand and bubble a lot during this time.
Remove from the heat, and allow to cool for another 10 minutes. When your mixture has cooled off, pour it into a mixing bowl, add your flavoring and food coloring, then whisk with your mixer on high until it becomes light and frothy. Typically, this will take between 5-10 minutes to become light, fluffy, and creamy looking.
For this next step, you may choose to pipe your marshmallow, or simply use a spoon to place it into the ice cream cones and shape the marshmallow. To pipe your filling, place your filling into a piping bag, then squeeze into your cones. You may also use a spoon to scoop the filling into your cones, then shape it to look like ice cream. If desired, you may add any toppings you wish, then serve and enjoy!