Preheat your oven to 350°F (175°C). Grease a mini muffin tin or line it with mini muffin liners.
In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.
In a large mixing bowl, combine the granulated sugar, brown sugar, and melted butter. Mix until smooth.
Add the eggs one at a time to the sugar mixture, whisking well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined. Do not overmix.
Fold in the chocolate chips until evenly distributed throughout the batter.
Fill each muffin cup about halfway with brownie batter.
Add a few mini marshmallows on top of each brownie batter cup. Sprinkle crushed graham crackers over the marshmallows.
Bake in the preheated oven for 10-12 minutes, or until the edges look set and a toothpick inserted into the center comes out with moist crumbs (not wet batter).
Once baked, remove from the oven and let them cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
Optional: If you want toasted marshmallows, switch on the broiler for 1-2 minutes to brown the tops slightly; keep an eye to prevent burning.
Notes
For toasted marshmallows, use the broiler for a minute or two after baking.