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 large mixing bowl, combine graham cracker crumbs, cocoa powder, sugar, melted butter, and salt. Mix until the mixture resembles wet sand.

Press the crumb mixture evenly into the bottom of the prepared baking dish to form the crust. Bake in the preheated oven for 10 minutes.

Remove the crust from the oven and pour the sweetened condensed milk evenly over the top.

Sprinkle mini marshmallows and chocolate chips over the condensed milk layer. If using, add chopped walnuts on top.

Return to the oven and bake for an additional 15-20 minutes, or until the marshmallows are golden brown and the chocolate is melted.

Allow the bars to cool completely in the pan before cutting into squares. For best results, chill in the refrigerator for at least 1 hour before serving.