Preheat oven to 375F. Line a large baking sheet with parchment paper.
Lightly roll out any crimps in the puff pastry sheets on a lightly floured surface. Working quickly to keep the puff pastry cold, slice into 12 equally sized rectangles (to make 6 pop tarts).
Roughly break up the chocolate bar and place 3-4 unevenly divided pieces into the centers of 6 rectangles. Brush with edges with a little water. Top with one of the remaining puff pastry rectangles, then crimp the edges to seal together with a fork. Repeat with remaining pop tarts.
Place on the prepared baking sheet. Brush with an egg wash, then use the fork to poke three small holes in the very top puff pastry layer.
Bake for 22-28 minutes, or until golden brown and puffed.
For the Marshmallow Fluff:
In the meantime, make the marshmallow fluff by adding the egg whites, sugar, and cream of tartar to the bowl of a stand mixer.
Fill a small saucepan with 2" of water, then bring to a boil (or use a double boiler).
Place the mixing bowl over the saucepan, whisking continuously until the sugar has dissolved, about 3-4 minutes. (You can test if sugar is dissolved by rubbing a little bit between your fingers and feeling for any graininess.)
Remove the mixing bowl from the saucepan and use the stand mixer to whisk on medium high speed for 5-8 minutes, or until soft peaks and fluffy. Fold in the vanilla extract.
Transfer to a piping bag.
Pipe a swirl of the marshmallow fluff onto the cooled pop tarts, then use a kitchen torch until the meringue has turned a golden brown. (Alternatively, you can broil for 1-2 minutes in the oven. Keep a close eye.)
Garnish with crushed graham crackers and finely chopped chocolate. Enjoy!