Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper for easier removal.
In a medium bowl, combine the graham cracker crumbs and melted butter. Mix until the crumbs are evenly coated. Press the mixture firmly into the bottom of the prepared baking dish to form a crust.
Evenly sprinkle the shredded coconut over the crust.
Next, add the chocolate chips and mini marshmallows on top of the coconut layer. If using, sprinkle the chopped nuts over the marshmallows.
Pour the sweetened condensed milk evenly over the entire mixture, ensuring all ingredients are moistened.
Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and the marshmallows are slightly toasted.
Allow the bars to cool completely in the pan before cutting into squares. For best results, chill in the refrigerator for about 1 hour before slicing.