Steam sweet-potatos until they are fork-tender. Then churn potatos through a food-mill on a course setting while still hot. Add butter and allow to melt while mixing. Add brown sugar and salt and mix. Finally add sour cream. Smooth mixture into a deep and wide caserole dish allowing at least one inch from top rim. Prior to serving bake at 350 for 30 minutes or so until hot. Cover evenly with mini-marshmallows and broil for 2-3 minutes until marshmallows melt together and form a golden-brown crust. Serve immediately.