Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish or line it with parchment paper for easy 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 graham cracker crust.
Layer the chocolate chips and mini marshmallows on top of the coconut.
If using, sprinkle the chopped nuts over the marshmallows and chocolate chips.
Pour the sweetened condensed milk evenly over the entire mixture, ensuring it covers all the ingredients.
Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and the center is set.
Remove from the oven and allow to cool completely in the pan before cutting into squares.