My mom makes one that everyone has to have every holiday. I will ask her what she puts in it. I know that she does the usual sugar, milk, butter and chocolate syrup on the heat until it forms a ball when it is dripped into cold water with a spoon. Then she takes it off the heat and whips in the peanut butter and then very quickly the marshmallow fluff with a spoon. If she cooks the first mixture too long then the fudge is very hard. If she does it just right then it is so creamy and just the right consistency. If she undercooks it then it does not set. So...it's all in the cooking time for that first mixture.