Preheat the oven to 190°C and line a baking tray with parchment paper.
On a lightly floured surface, roll out the puff pastry dough into a 12-inch square, about 1/4 inch thick. Trim the edges and then cut the puff into 4 squares and place them onto the prepared baking tray.
Place 1 biscuit in the centre of each pastry square and sprinkle neatly with half of the marshmallows and chocolate chunks or chips. Top this with the remaining graham crackers. Fold the pastry to just come over the biscuit and press the corners and edges to secure (although the tart will open up a bit as it bakes – that is to be expected.
Brush the puff pastry with the eggwash and bake for 15 minutes. Pull the tarts from the oven and top the tarts with the remaining marshmallows and chocolate and return the tarts to the oven to bake for another 10-12 minutes, until the pastry is golden brown. The tarts are best served warm, but allow them to cool for 15 minutes before serving.