Preheat Oven: Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

Mix Dry Ingredients: In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.

Cream Butter and Sugar: In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add Egg and Vanilla: Beat in the egg and vanilla extract until well combined.

Combine Mixtures: Gradually add the dry ingredients to the wet mixture, mixing until just combined. If using, fold in the mini marshmallows and chocolate chips until evenly distributed.

Scoop Cookies: Using a cookie scoop or tablespoon, drop rounded balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.

Swirl Marshmallows: Gently press a few additional mini marshmallows on top of each cookie and swirl them into the dough for a decorative touch.

Bake: Bake in the preheated oven for 10-12 minutes, or until the cookies are set but still soft in the center.

Cool: Remove from the oven and let the cookies cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.