Whisk 1/2 cup heavy cream (ideally extra-cold), 1/4 cup marshmallow fluff and 1/8 teaspoon vanilla extract until stiff peaks form; set aside. Combine 1 cup cold whole milk and 1/4 cup chocolate sauce in a blender. Add 3 cups chocolate ice cream; blend until smooth. Fill 4 glasses one-quarter of the way with the shake; add some marshmallow cream, then more milkshake, then cream, then milkshake. Finish with a dollop of marshmallow cream and garnish with grated bittersweet chocolate.