Preheat the oven to 350 F and prepare an 8x8 inch pan with parchment paper. Arrange the graham crackers evenly into the bottom and set aside.
In a small bowl, stir together the flax meal and water and then set it aside to gel for 5 minutes. Meanwhile, in a bowl, stir together the oat flour, tapioca starch, gum, baking soda and salt. Set it aside. In another bowl, use a handheld mixer to beat the butter, brown sugar and cane sugar until light and fluffy, about 1-2 minutes. Add in the flax mixture and vanilla extract and continue mixing until well combined. Add the dry ingredients to the wet and mix until a cookie dough forms. Fold in the chocolate chips and half of the chopped chocolate.
Use cookie scooper or spoon to drop half of the cookie dough over the crackers in the prepared pan and spread it evenly to the edges of the pan. Cut the marshmallows into small pieces and press them into the cookie in the pan. Top with ramining cookie dough and chocolate pieces. Bake the s'mores bars for 12-15 minutes. The edges should be golden brown and the center will look just slightly undone. Remove it from the oven and let it cool for at least 30 minutes before cutting into bars and serving.
Store these bars in an airtight container in the refrigerator for up to 5 days and in the freezer for 1 month.