Preheat the oven to 375 degrees Fahrenheit and line two baking sheets with parchment paper.
Add the pieces of chocolate bar and butter to a microwave-safe bowl. Melt in 30-second intervals, stirring at each interval before returning to the microwave, until it’s completely melted and smooth. Set aside to cool slightly.
Sift the flour, baking powder, salt and cocoa powder into a medium bowl. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment or with an electric hand mixer, beat the eggs, brown sugar, vanilla and peppermint extracts until well combined.
Slowly, pour in the melted chocolate and beat until incorporated. Scrape down the sides of the bowl as needed.
With the mixer set on low speed, gradually add the dry ingredients and mix until just combined.
Fold in the chocolate chunks and mix until they are well distributed.
Remove the dough from the mixer and cover it in plastic wrap. Chill in the refrigerator for at least 30 minutes.
Once chilled, scoop dough out about 1 heaping tablespoon at a time. Shape into balls of dough and arrange on the prepared baking sheets leaving 2-inches between each cookie.
Bake until slightly under-baked, about 9 minutes.
In the meantime, cut the marshmallows in half using kitchen scissors. Place peppermint candy in a resealable bag and crush them up with a rolling pin.
Remove the cookies from the oven and place the marshmallow half placing the cut side down on top of each cookie and return to the oven.
Bake for about 2-3 minutes or until the marshmallows are melted.
Transfer the cookies to a wire rack to cool slightly then, drizzle the cookies with chocolate and top with the crushed peppermint. Enjoy!