Preheat your grill to 350°F for indirect cooking. For a 3-burner grill: Turn the two outside burners to medium and leave the center burner off. Place the skillet over the center burner. For a 4-burner grill: Turn the two outside burners to medium and leave the two center burners off. Place the skillet in the center of the grill.Place the skillet in the center of the grill.If using an oven, follow instructions for the brownie mix. Grease a 10-inch cast iron skillet.
In a large bowl, combine the brownie mix, eggs, oil, Kahlúa (or coffee/milk), and espresso. Stir just until combined, then fold in the chocolate chips.
Pour the batter into the prepared skillet and smooth the top.
Place the skillet on the cooler side of the grill, close the lid, and bake for 30–40 minutes, or until the edges are set and the center is just barely fudgy.
Remove the skillet briefly and cover the top with an even layer of mini marshmallows. Sprinkle over the additional chocolate and crushed graham crackers.
Return to the grill for 2–5 minutes, just until the marshmallows are puffed and golden. Watch closely—they toast quickly.
Let cool for about 10 minutes before serving.