Mix cooked potatoes with butter and brown sugar, salt and 1/2 cup crushed cornflakes. Heap potato mixture about 1/3 cup, onto each slice of pineapple. Push a marshmallow into each heap and sprinkle with crushed cornflakes. Place on a baking sheet and bake for about 10 minutes at 350 F. or until marshmallow pops open and they are slightly browned.