Preheat your oven to 375°F (190°C) and line two baking sheets with parchment paper.

In a microwave-safe bowl, combine the semi-sweet baking chocolate and unsalted butter. Heat them in the microwave for 30-second intervals, stirring in between, until smooth and fully melted. Allow this mixture to cool slightly for about 5 minutes.

In a separate bowl, sift together the all-purpose flour, baking powder, salt, and Dutch-process cocoa powder.

Using a mixer, beat together the eggs, light brown sugar, vanilla extract, and peppermint extract until the mixture is light and fluffy, about 3–4 minutes.

Gradually mix the cooled chocolate and butter mixture into the egg mixture.

Slowly add the sifted dry ingredients to the wet mixture and mix on low speed until just combined. Gently fold in the chocolate chunks.

Cover the bowl with plastic wrap and refrigerate the cookie dough for at least 30 minutes.

Use a cookie scoop to portion the dough into balls, about 2 tablespoons each, and place them on the prepared baking sheets, spaced about 2 inches apart.

Bake the cookies for approximately 9 minutes. The edges should look set while the centers remain soft and slightly underbaked.

Remove the cookies from the oven and immediately place a half marshmallow on top of each cookie. Return to the oven for 2–3 minutes until the marshmallows are softened and toasted.

Transfer the warm cookies to a wire rack to cool completely. Drizzle with optional chocolate sauce and sprinkle crushed peppermint candy over the marshmallow-topped cookies.

Once cooled, these cookies are ready to be enjoyed warm or at room temperature.