Add ¼ cup (60 ml) milk, 2 scoops of ice cream, and 6 tablespoons chocolate syrup to a blender. Blend until smooth, then divide between 2 tall glasses. Rinse out the blender.
Add ¼ cup (60 ml) milk, 2 scoops of ice cream, and the remaining 2 tablespoons chocolate syrup to the blender. Blend until smooth, then pour into the glasses over the first chocolate layer. Rinse out the blender.
Add the remaining ¼ cup (60 ml) milk and 2 scoops of ice cream to the blender. Blend until smooth, then pour into the glasses over the second layer.
Top the milkshakes with the chocolate shavings and graham cracker crumbs.
Thread the marshmallows onto 2 wooden skewers and toast with a culinary torch until browned all over. Set on top of the glasses.