Preheat the oven to 350°F and line a muffin tin with 4 cupcake liners. In a medium bowl, whisk together the flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt.

Add the milk, vegetable oil, egg, and vanilla extract to the dry ingredients. Stir until the batter is smooth and no dry streaks remain.

Divide the crushed graham crackers evenly among the 4 liners to form a thin base layer. Spoon the batter over the crumbs, filling each liner about two-thirds full. Sprinkle the mini chocolate chips on top of the batter.

Bake for 18 to 20 minutes until a toothpick inserted in the center comes out clean. Remove the cupcakes from the oven and let them cool completely in the tin.

In a small bowl, mix the marshmallow fluff with the powdered sugar until smooth. Spread or pipe the mixture over each cooled cupcake. Top with the mini marshmallows.

Place the cupcakes under the broiler for 30 to 60 seconds, watching closely, until the marshmallows turn golden brown. Serve once the topping has set slightly.