Soak the gelatine in cold water.
If you are using fresh mangoes, peel, cube, then process then until you have a puree.
In a saucepan, add the mango puree, the sugar, and the corn starch. Mix and cook until they start bubbling.
Squeeze the gelatine sheets to get rid of all the water, add them to the hot puree, and mix. They will immediatly melt.
While the puree is still hot, pour it in a round mold or a circle with 18cm diameter (for the mold and shape I am using). You can use any silicon mold or shape you prefer.
Freeze the puree for at least 6 to 8 hours.