Using a food processor or even a blender, crumb the graham crackers finely so they are completely ground. Mix them with all purpose flour and add the frozen butter cubes.Using your fingers, mix the butter and flour mixture together until resembles coarse breadcrumbs.

Add in water one spoon at a time and bring the mixture together to form a smooth ball of dough. Shape it into a disk and wrap it in a plastic wrap. Chill it in the fridge for 30 minutes.

Preheat the oven to 350F. Line a baking sheet with a parchment paper.

Coat the surface lightly with flour and roll out the dough to a ½ inch thickness.

Sprinkle 1 tablespoon sugar on the crust. Break the graham crackers into small pieces and place them on the crust, leaving 2 inch around the edge. Top with half of the dark chocolate.

Fold the edges up and crease and overlap the crust so it sticks together. beat the egg and brush the edges with egg. Sprinkle sugar generously on the edges.

Bake in the oven for 30 minutes until the crust is golden and cooked.

Place the remaining chocolate on the galette and top with marshmallows. Turn the broiler on and broil for 3-5 minutes until marshmallows are roasted as you like them.

Let the galette cool for 5 minutes and then cut it.