In a large microwave-safe bowl, combine marshmallows and milk. Microwave in 30-second intervals, stirring in between until the marshmallows are completely melted (about 2 minutes total). Stir in the raspberries and then fold in the Cool Whip.

Pour the mixture into the crust and either refrigerate or freeze. We prefer the refrigerated texture - it only takes a couple of hours to set up nicely and then it's ready to eat!