Preheat your oven to 350°F (175°C).
Prepare a 9x13-inch baking pan by greasing it lightly or lining it with parchment paper.
In a mixing bowl, combine graham cracker crumbs, cocoa powder, sugar, and melted butter. Mix well until the texture resembles wet sand.
Press the mixture firmly into the bottom of the prepared pan to form an even layer.
Bake the crust in the preheated oven for 10 minutes, then remove to cool slightly.
Sprinkle chocolate chips evenly over the crust, followed by an even layer of mini marshmallows.
Return the pan to the oven and bake for an additional 15 minutes, or until the marshmallows are golden brown.
Allow the bars to cool in the pan before cutting into squares and serving.