Drain out about 1/4 of the sweet potato juice and put in a sauce pan. Add the pineapple chunks with about half the juice. Add the brown sugar and the cinnamon. Mix the cornstarch with about 1/4-cup of the pineapple juice and add to the sauce pan. Cook over med to high heat, stirring until bubbling and starting to thicken. Stir in the pecans if using them.
Pour in to 8 by 8 glass baking pan. Place the pineapple slices on top of the potato mixture. Sprinkle with the marshmallows.