Step 1: Prepare the Dry Ingredients

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

Step 2: Cream the Butter and Sugars
2. In a large mixing bowl, use a mixer to cream the softened butter, granulated sugar, and brown sugar until light and fluffy, about 2-3 minutes.

Step 3: Add the Wet Ingredients
3. Beat in the egg and vanilla extract until well combined.
4. Mix in the milk until smooth.

Step 4: Combine Wet and Dry Ingredients
5. Gradually add the dry ingredients to the wet mixture, mixing until just combined.

Step 5: Fold in Marshmallows and Chocolate Chips
6. Gently fold in the mini marshmallows and chocolate chips. Be careful not to overmix.

Step 6: Chill the Dough
7. Cover the bowl with plastic wrap and chill the dough in the refrigerator for at least 30 minutes.

Step 7: Preheat the Oven and Shape the Cookies
8. Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.
9. Scoop out tablespoon-sized portions of dough and place them on the prepared baking sheets, about 2 inches apart.

Step 8: Bake the Cookies
10. Bake for 8-10 minutes, or until the edges are set but the centers are still soft. The marshmallows will melt and create swirls in the cookies.

Step 9: Cool and Enjoy
11. Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.