Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the graham cracker crumbs, sugar, cocoa powder, baking powder, and salt. Mix well.
Pour in the melted butter, milk, and vanilla extract. Stir until just combined. Do not overmix; some lumps are okay.
Grease a 9-inch (23 cm) baking dish and pour the batter evenly into the dish.
Sprinkle the chocolate chips evenly over the batter, followed by the mini marshmallows and chopped pecans if using.
Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the marshmallows are toasted.
Remove from the oven and let it cool for 10 minutes before serving. Enjoy warm for the best experience!