Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a large bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy.

Add the egg and vanilla extract, mixing until well combined.

In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, stirring until a dough forms. Fold in the chocolate chips and mini marshmallows.

Scoop rounded tablespoons of dough onto the prepared baking sheet, spacing them about 2 inches apart.

Bake for 10-12 minutes, or until the cookies are set around the edges but still soft in the center. The marshmallows should melt and create a delicious swirl effect.

Let the cookies cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely. Enjoy your gooey Chocolate Marshmallow Swirl Cookies!