Preheat the oven to 180°C/356°F and line a 9×9 inch baking pan with parchment paper.
Cookie base: Combine flaxseed meal and hot water together to create a flax egg. Set aside. Beat together the vegan butter and light brown sugar until light and smooth. Add in the vanilla extract and flaxegg. In a food processor, blitz the graham crackers until they become fine crumbs. Transfer the fine crumbs into a medium-sized bowl with the all-purpose flour. salt and baking powder. Mix, and then add to the bowl with the wet ingredients. Mix together until well-combined. Tightly press 3/4 of the cookie mixture into the pre-lined baking tray using clean hands.
The layers: Add the chunks of vegan dark chocolate over the bottom layer, and then, add the vegan marshmallows over them. With the remaining dough, grabbing a little at a time, press the dough between your palms and gently flatten them. Place over the marshmallow layer. Bake for 30-40 minutes or until the cookie tops are golden and the marshmallows start to become toasted.
Final touches: Remove from the oven, and sprinkle with sea salt flakes (or regular salt, just use way less!). Allow them to cool for 10 minutes. Slice and enjoy!