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

In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and brown sugar for about 2-3 minutes until light and fluffy.

Beat in the eggs and vanilla extract until well combined.

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

Gradually fold the dry ingredients into the wet mixture until just combined.

Fold in the semi-sweet chocolate chips if desired.

Cover the dough and chill it in the refrigerator for at least 30 minutes.

Flatten a tablespoon of dough, place Nutella and marshmallow fluff in the center, and enclose the filling with the dough.

Arrange filled dough balls on the baking sheet and bake for 10-12 minutes.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack.