Preheat the oven to 325℉.
Spray the muffin pan with cooking spray and dust it with flour coating the pan liberally (if you are using muffin cups, lightly spray the muffin pan and place the muffin cups into the sprayed pan).
In the small skillet melt the butter and set it aside.
Place brown sugar, white sugar, melted butter, and vanilla extract in the large bowl. Using a wooden spoon mix to combine the ingredients.
Add the eggs and stir to combine.
Then add the unsweetened cocoa powder and stir to combine using a wire whisk.
Then add the baking powder and salt and stir to combine using the wire whisk.
Followed by the all-purpose flour, continue to stir to combine using the wire whisk, until the batter is smooth.
Fold in the dark chocolate chips until evenly combined.
Using a cookie scoop or a large spoon, scoop the batter into the prepared muffin pan filling it ½ way in each slot, and place the mini marshmallows on the top of each brownie,
Place on the center rack of a preheated oven for 22 minutes. The marshmallows will be toasted and melted. Place the pan onto a wire rack to cool for 10-15 minutes. Then using an offset spatula loosen the brownies and remove them from the pan and place the brownies on the wire rack to continue cooling. If you are using muffin cups, then just pull the brownies from the pan and place them on the wire rack to cool.
To serve sprinkle the brownies with additional mini marshmallows and chocolate chips.
