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. Place in prepared pie crust. Chill at least 2 hours or until set.
Serve with additional whipped cream (optional).
Store in the refrigerator for up to 2 days. I do not suggest freezing this pie.
Notes
Don't skip the cooling step - you cannot mix the whipped cream into the marshmallow mixture until it's completely cool or it will melt.
Top this however you like - with more whipped cream or anything at all.
Use any kind of crust you like - homemade or store bought.