Prepare the Dough : In a medium bowl, whisk together the flour, baking powder, salt, and cocoa powder. Set aside. In a large heatproof bowl, melt the butter and 12 oz of semi-sweet baking chocolate together in the microwave or using a double boiler. Stir until smooth and let cool slightly. Add the brown sugar to the melted chocolate mixture and stir to combine. Beat in the eggs, one at a time, followed by the vanilla and peppermint extracts.

Combine Wet and Dry Ingredients : Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in the chocolate chunks. The dough will be thick. Cover the bowl with plastic wrap and refrigerate for 30 minutes to firm up.

Bake the Cookies : Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper. Scoop about 2 tablespoons of dough for each cookie and roll into balls. Place them on the prepared baking sheets, spacing them about 2 inches apart. Bake for 10–12 minutes, or until the edges are set but the centers are still soft.

Add the Marshmallows : While the cookies are still warm, press one large marshmallow into the center of each cookie. Return the cookies to the oven and bake for an additional 2 minutes, or until the marshmallows puff up slightly.

Cool and Serve : Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Optional Decoration : For an extra festive touch, drizzle melted chocolate or crushed candy canes over the cookies before serving.