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.

Preheat the oven to 425 degrees F.

Place a sheet of parchment paper on your work surface. Place one sheet of puff pastry on top. Spoon 1 to 2 teaspoons of marshmallow in 9 spots on the pastry – sort of like a tic tac toe board. Spoon 1 to 2 teaspoons of nutella on top. Finish off with a square of chocolate. Cover the entire thing with the other sheet of puff pastry. Use a large knife or pizza cutter to cut the pastry into 9 squares, just around the edges. Use a fork to press the edges together.

Brush the tops with the beaten egg wash. Bake for 20 to 25 minutes, or until golden brown. Let cool slightly, then cover in nutella glaze. These are best served shortly after making.

Make this while the tarts are baking! Whisk together until the nutella is smooth and… drippy. If it is still thick, add more cream 1 teaspoon at a time, whisking well. Pour the glaze over the pastries and sprinkle with graham crumbs.