Combine egg whites, sugar and cream of tartar in a heat proof bowl set over a pot of gently simmering water (I used the stand mixer bowl to reduce amount of bowls needed).
Heat the mixture, while stirring constantly, until an instant thermometer reads 160 degrees, about 5 minutes.
Remove the bowl from the heat and using the whisk attachment of your stand mixer, mix on high speed until stiff, shiny peaks form, about 5 min. Mix in the vanilla.
Decorate the top of the tart with the marshmallow fluff. If desired, use a kitchen torch to toast the top of the marshmallow fluff.
Refrigerate the tart for 2 hours prior to serving.