Line a large baking sheet with parchment paper. Set aside.
In a medium bowl, whisk together the flour, cocoa powder, salt, and baking powder. Set aside.
To the bowl of a stand mixer fitted with the paddle attachment, add the butter, brown sugar, and granulated sugar. Mix on medium speed until creamy.
Add the egg yolks and peppermint extract. Mix until well incorporated, scraping the bowl as needed.
Reduce the mixer speed to low and gradually add the dry ingredients to the wet ingredients, mixing until just combined. Avoid overmixing, as this can affect the cookie texture.
Using a 1-tablespoon cookie scoop, drop the dough onto the prepared baking sheet. Chill in the refrigerator for 1 hour.
Once chilled, preheat the oven to 350°. Line 2 large baking sheets with parchment paper.
Transfer the dough balls to the lined baking sheets, spacing the dough balls about an inch apart. Using a small teaspoon or your thumb, press an indent into the middle of each dough ball.
Bake for 9 minutes.
Remove cookies from the oven and add 1 marshmallow half to each cookie, placing the cut side down in the indent. Return to oven and bake an additional 2-3 minutes, or until the marshmallows have begun to melt.
Remove from oven and gently press down on marshmallows to flatten slightly.
Cool on the pan for several minutes then transfer the cookies to a wire rack to finish cooling.
While the cookies cool, melt chocolate. To a small microwave-safe bowl, add the chocolate chips. Microwave in 30-second intervals, stirring until melted.
Transfer the melted chocolate to a piping bag or a small zip-top bag with the corner snipped off, then drizzle over the cooled cookies.
Sprinkle each cookie with crushed peppermint candies.
Let chocolate set before serving.