Preheat your oven to 350°F (175°C). Line a 9x9 inch baking pan with parchment paper, leaving some overhang for easy removal.
In a mixing bowl, combine the graham cracker crumbs and cocoa powder. Pour in the melted butter, and stir until the mixture resembles wet sand.
Press the crumb mixture evenly into the bottom of the prepared baking pan to form a crust. Bake in the preheated oven for 10 minutes.
Remove from the oven and immediately drizzle the sweetened condensed milk over the crust.
Sprinkle the semi-sweet chocolate chips and mini marshmallows evenly on top of the condensed milk layer. If using, add the crushed walnuts for added crunch.
Return the pan to the oven and bake for an additional 15 minutes, or until the marshmallows are golden and the chocolate is melted.
Let the bars cool completely in the pan on a wire rack, then lift them out using the parchment paper overhang. Cut into squares and serve.
Notes
Arrange the bars on a colorful plate and sprinkle a few extra mini marshmallows on top for a garnish. Consider serving them with a side of chocolate sauce for dipping!