Prepare the Dough

1. Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
2. In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.
3. Add the eggs, one at a time, beating well after each addition. Mix in the vanilla extract.
4. In a separate bowl, sift together the flour, cocoa powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients and mix until combined.
5. Fold in the semi-sweet chocolate chips. Chill the dough in the refrigerator for 20–30 minutes to make it easier to handle.

Stuff the Cookies

6. Scoop 2-tablespoon portions of chilled dough and flatten them into discs.
7. Place 1/2 teaspoon of Nutella and 1/2 teaspoon of marshmallow fluff in the center of each disc.
8. Gently fold the edges of the dough around the filling, sealing it completely. Roll the dough into a ball and place it on the prepared baking sheet. Repeat with the remaining dough and filling, leaving about 2 inches between each cookie.

Bake the Cookies

9. Bake the cookies for 10–12 minutes, or until the edges are set but the centers are still slightly soft.
10. Remove from the oven and allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.