Preheat oven to 175°C (350°F) and grease an 8x8 inch square baking pan.

In a large mixing bowl, whisk together the melted butter and granulated sugar until well combined.

Add eggs and vanilla extract to the butter mixture and whisk vigorously until smooth.

Sift the cocoa powder, flour, salt, and baking powder into the wet ingredients.

Fold the dry ingredients into the wet ingredients using a spatula until just combined; do not overmix.

Gently fold in the chocolate chips.

Pour the batter into the prepared baking pan and spread evenly.

Bake for 22 to 25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Remove the pan from the oven and immediately cover the surface with an even layer of mini marshmallows.

Set the oven to broiler mode and place the pan back in for 1 to 2 minutes until the marshmallows are golden brown and toasted.

Allow the brownies to cool completely in the pan before slicing into squares.