Is this it? My sister in law was really big into Tupperware in the late 70's and we would have game night once a week. She always made the caramel corn. I have this recipe from her written on the inside of my Joy of Cooking cookbook.
Put popped corn in a large Tupperware bowl. In a heavy saucepan, place 18 large marshmallows, 1 stick margarine, and about 1/3 c. brown sugar. Cook the marshmallows, margarine, and brown sugar, stirring constantly. The mixture will foam up. Pour over the popped corn and mix up with a large spoon. Serve while warm. It will be gooey and good.