In a stand mixer or with a hand-held mixer, beat the heavy cream with the marshmallow fluff until stiff peaks form. Set the whipped marshmallow cream aside.

Put the milk in a blender, then the ice cream. Blend it together, and check the consistency. If you want a thinner shake, add a tiny bit more milk and blend again. Throw two fun-sized candy bars into the blender. Pulse two or three times, just enough to sort of chop up the candy, but not to puree it. Pour the milkshake into a glass, and top with a big dollop of the marshmallow cream and another chopped-up fun-sized candy bar.