Fill a sauce pan with about two inches of water and bring it to a gentle simmer. Separate the yolks from the egg whites and make sure NONE of the yolk gets into your mixture.
In a heat proof bowl, place the egg whites, powdered sugar and cream of tartar. Place the bowl on the sauce pan over the simmering water but do NOT let the bottom of the bowl touch the simmering water. You can also use a double boiler if you have one.
Continuously whisk while the mixture heats up. It will start out thick, and the powdered sugar may be clumpy (this is okay!), it will thin out and the clumps will dissipate as you keep on whisking. Whisk for about 4-5 minutes until the mixture is thin and frothy on top - if you have a thermometer, it should be around 160 degrees Fahrenheit.
Remove from the heat and add the vanilla extract. Using a handheld mixer or in the bowl of a stand mixer fitted with the whisk attachment, beat on high speed for 5-6 minutes until luscious, puffy, stiff peaks form.
Once it is finished, you can either dollop or pipe a bit of the marshmallow fluff onto the chocolate filling. If you have a baking torch, I highly suggest toasting the marshmallow fluff topping a bit - it LITERALLY tastes like a fireside marshmallow.