Place marshmallows and milk in a medium saucepan and heat over medium heat. Stir constantly until marshmallows are completely melted. Remove from heat to cool completely before continuing.

Place heavy whipping cream and vanilla extract in a large bowl (or the bowl of a stand mixer). Beat until stiff peaks form.

Gently fold the whipped cream into the marshmallow mixture. Pour into the prepared pie crust. Chill in the refrigerator for at least 2 hours or until set.

Serve with optional additional whipped cream.