Wash potatoes; pierce with a fork. Place in ring formation in microwave on high for 10-14 minutes, turning potatoes after 5 minutes. Test for doneness after 8 minutes and remove any potatoes that are soft. Continue cooking until all potatoes are soft. Wrap potatoes in paper towels and cool for 5 minutes. Split potatoes in half and scoop out flesh. Whip potatoes with butter, brown sugar and salt until smooth. Spoon into 1-1/2 quart casserole; spread evenly. Spoon pie filling over potatoes. Place marshmallows in ring on top of pie filling. Bake at 350 degrees 30-45 minutes until heated and marshmallows are lightly browned.