Unwrap all your Hershey bars, break them up and place them in a nonstick pot over low-medium heat.
NOTE: If you don't have a nonstick pot, then you'll want to do this in a double broiler. Just take a heatproof bowl and place it on top of a saucepan that has simmering water in it. This will help the chocolate to melt slowly without it sticking.
Add in marshmallows and milk.
Stir constantly until marshmallows and chocolate are melted and smooth.
Take off heat and allow to cool. Be sure to stir several times while cooling. It will thicken slightly as it cools (about 15 minutes).
Once cooled, fold in the whipped topping. Don't over stir. A few times around should be good enough.
Pour into pie crust.
Put it in the refrigerator to finish setting up for at least 3-4 hours or pop it into the freezer.