Preheat the oven to 250 degrees F (120 degrees C).
Mix graham cracker crumbs and sugar together in a bowl. Add melted butter and toss until combined. Press mixture into the bottom of a 9x13-inch baking pan in an even layer.
Spread chocolate chips evenly over the graham cracker mixture. Sprinkle walnuts over chocolate chips.
Lay 7 graham crackers evenly into the pan, then pour the mini marshmallows on top of the graham crackers in an even layer. Lay remaining graham crackers over marshmallows, then drizzle chocolate syrup on top.
Bake in the preheated oven for 30 minutes. Let cool at least 30 minutes before cutting and serving.