In a large mixing bowl add the roasted chestnuts, sugar, and rum. Carefully, light the content with a match to flambe the ingredients. Stir continuously to dissolve the sugar. Continue to stir until the rum has reduced to a thick syrup or is caramelized and the flame has died off.
To make Roasted Chestnuts: Using a sharp paring knife, make a slit on the rounded side of each chestnut. Place the chestnuts on a perforated pan and roast over a fire for 10 to 15 minutes. Alternatively cook in a preheated oven at 400 degrees F for 15 to 20 minutes. Peel the charred shell and serve.
The slit will prevent the chestnuts from exploding while cooking.