In a medium microwave-safe bowl, place the broken chocolate bars and pour the milk over the top of the chocolate.
Microwave one minute, then stir and microwave at 15-second intervals until the chocolate and milk are smooth.
Add the marshmallows and microwave at 15-second intervals, stirring between each, until marshmallows are fully melted and incorporated.
Let the mixture cool on the counter, stirring occasionally, until room temperature. Or, to cool faster, place in the refrigerator, stirring every 10 to 15 minutes until room temperature, about 1 hour. Keep an eye on it, so it doesn't solidify.
Once the mixture is at room temperature, whip the cream on medium-high speed until stiff peaks form.
Fold the whipped cream into the melted chocolate until fully combined and smooth.
Pour the mixture into the pie shell.
Cool for at least 3 hours in the refrigerator before serving. Add whipped cream on top before serving, if desired.