On a lightly floured work surface, roll the dough out to a 17 x 6-inch (42 x 15 cm) rectangle. Line the bottom and sides of a 14 x 4-inch (35.5 x 10 cm) rectangular tart pan with removable bottom with the dough, trimming off any excess. With a fork, prick the dough all over. Refrigerate for 30 minutes or freeze for 15 minutes.
With the rack in the bottom position, preheat the oven to 400°F (200°C).
Bake for 15 to 18 minutes or until the crust is nicely golden. Let cool on a wire rack.
In a bowl, sprinkle the gelatin over the water. Let bloom for 5 minutes.
In a pot, heat 1/4 cup (60 ml) of the maple syrup until it starts to smoke and darken slightly. Quickly add the remaining maple syrup and the corn syrup. Bring to a boil and simmer for 1 minute. Remove from the heat. Add the gelatin and whisk until dissolved. Let cool for 15 minutes.
With an electric mixer, beat the mixture until it triples in volume and forms stiff peaks. Pour the marshmallow mixture into the cooled crust and spread into an even layer. Let cool completely at room temperature.
Cover the top of the marshmallow with the melted chocolate, tipping the pan from side to side to evenly distribute the chocolate. Let the chocolate harden in the refrigerator for 1 hour or at room temperature for 3 hours. Cut into bars and serve. The chocolate bars are best enjoyed on the same day they’re made.