In a large mixing bowl, cream together ½ cup of softened butter, ½ cup of brown sugar, and ½ cup of granulated sugar until light and fluffy, about 3-5 minutes.

Beat in 1 large egg and 1 teaspoon of vanilla extract until well combined.

Gradually add in 1 cup of all-purpose flour and ⅓ cup of cocoa powder, mixing until a thick, smooth dough forms.

Gently fold in 1 cup of chopped mini marshmallows into the chocolate dough, ensuring they are evenly distributed.

If your dough feels sticky to the touch, wrap it in plastic wrap and refrigerate for about 30 minutes.

Preheat your oven to 350°F (175°C).

After chilling, scoop out dough portions using a cookie scoop or tablespoon, rolling them into balls.

Place these dough balls onto a baking sheet lined with parchment paper, spacing them about 2 inches apart.

Bake the cookies in your preheated oven for 10-12 minutes, rotating halfway through for even baking.

Once baked, allow the cookies to cool on the baking sheet for 5 minutes before transferring to wire racks.