To make the topping, combine the egg whites, sugar and cream of tartar in a heat-proof bowl, preferably the bowl of your electric stand mixer. Place over top of a double-boiler that contains simmering water, and whisk constantly for 3-4 minutes until the sugar has dissolved and the egg whites are slightly warm. Immediately remove the bowl and place it on your stand mixer with the whisk attachment, beating slow at first and gradually increasing the speed to high. Beat for 6-7 minutes until glossy and thick, then beat in vanilla extract for another minute until combined. Spread the marshmallow on the tart with the back of a spoon. You can torch the frosting if you’d like, as well as cover the top in additional graham crumbs. I stored this in the fridge with a layer of plastic wrap loosely over top.