In a medium saucepan over medium heat, combine butter and chocolate chips and stir constantly until they're combined and just melted. Remove from heat and set aside to cool.

In a large bowl, combine egg, brown sugar and vanilla and beat on medium speed just until just combined. Add the cooled butter/chocolate mixture and mix on low until just combined.

Add flour, cocoa powder, baking powder and salt and mix on low until just combined. Cover the bowl with plastic wrap and place in the fridge to chill for at least 2 hours.

Preheat oven to 325° F. Line a baking sheet with a non-stick silicon mat and use a small cookie scoop to place dough onto prepared baking sheet a couple inches apart (they will spread quite a bit). Bake for 10-12 minutes (I found 11 to be perfect).

Promptly remove the cookies from the oven and place one half of an Andes mint on top of each cookie, and then place one half of a marshmallow on top of the mint.

Place the cookies back in oven and bake for another 3-4 minutes. When you remove them, gently press down on the top of the marshmallows just to flatten them ever so slightly. Let cool on pan for about 5 minutes and then move to a wire rack to cool completely.