Cover a rolling pin and the inside of a 3-qt. bowl with plastic wrap; spray with cooking spray. In a pot, melt butter over medium heat. Add mini marshmallows; stir to melt. Remove from heat and stir in rice cereal; let cool. Spray hands with cooking spray. Cover the inside of the bowl with 3/4 of the cereal mixture; shape the rest halfway around a 2-inch portion of rolling pin. Refrigerate both until firm. Lift plastic wrap to remove igloo from bowl; invert onto platter. Press smaller piece onto igloo as entryway. Cover with Cool Whip and sprinkles or candies.