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

2. Cream Butter and Sugars:
In a large bowl, cream together softened butter, granulated sugar, and brown sugar until light and fluffy.

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

4. Combine Dry Ingredients:
In another bowl, whisk together flour, cocoa powder, baking soda, and salt.

5. Make Dough:
Gradually add the dry ingredients to the wet mixture, stirring until fully incorporated. Fold in chocolate chips.

6. Add Marshmallows:
Gently fold in mini marshmallows, being careful not to overmix (this helps them stay gooey inside).

7. Scoop and Bake:
Scoop tablespoon-sized portions of dough onto the baking sheet. Bake for 9–11 minutes, or until edges are set but centers look slightly soft.

8. Cool and Enjoy:
Let cookies cool for 5 minutes on the baking sheet before transferring to a wire rack. Enjoy warm for maximum gooeyness!