In a small saucepan over low heat, combine butter and chocolate chips and stir constantly until they're combined and just melted. Remove from heat and set aside to cool.
1/4 cup unsalted butter, 6 ounces semisweet chocolate chips
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.
1 egg, 1/2 cup + 2 tablespoons light brown sugar, 1 teaspoon pure vanilla extract
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 1 hour or until it's not sticky and easier to work with.
3/4 cup all-purpose flour, 1/8 cup Dutch process cocoa powder, 3/4 teaspoon baking powder, 1/8 teaspoon salt
Preheat oven to 325° F. Line a baking sheet with a non-stick silicone 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.
9 Andes mints, 9 large marshmallows
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. Serve and enjoy!
Notes
Serving recommendation: These can be served at room temperature, or you can do what I do and warm them up in the microwave for a few seconds to let the marshmallow and chocolate mint get a little melty.Storage: Store at room temperature for up to 5 days.