Preheat your oven to 350°F (175°C).
Prepare a 9x9-inch baking pan by greasing it or lining it with parchment paper.
In a large mixing bowl, combine graham cracker crumbs, cocoa powder, brown sugar, and granulated sugar.
Pour in the melted butter and add the egg to the dry mixture; mix until thoroughly combined.
Press the mixture firmly into the bottom of the prepared pan, creating an even layer.
Sprinkle the chocolate chips evenly over the crust, followed by a layer of mini marshmallows.
Bake in the preheated oven for 15–20 minutes, or until the marshmallows are lightly browned.
Remove from the oven and let cool in the pan for about 10 minutes before slicing.