Put 4 cups of mini marshmallows into the freezer until they are frozen solid; set aside.
In a large bowl combine the broken Hershey's milk chocolate pieces, semi-sweet chocolate chips, and the butter pieces; set aside.
In a large saucepan combine the sugar, evaporated milk, and 2 1/2 cups of mini marshmallows (the amount listed to cook); stir to combine.
Bring the mixture to a boil over medium heat, stirring constantly; once it reaches a light boil, continue to cook and stir for 7 minutes.
Remove the pan from heat and immediately pour the hot sugar mixture over the chocolate and butter in the large bowl.
Stir with a wooden spoon until the chocolate is melted and the mixture loses its shine; it may look separated—keep stirring until it smooths as it cools.
Stir in the vanilla, then fold in the frozen 4 cups of mini marshmallows and the chopped walnuts or pecans.
Spread the fudge evenly into a buttered cookie sheet or into two buttered 9x13 baking dishes for a full batch.
Cool completely at room temperature or chill to speed up setting, then cut into pieces and serve.