In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy (about 3 minutes).

Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.

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

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fold in the semi-sweet chocolate chips. Cover the dough and refrigerate for at least 30 minutes to firm up.

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

Scoop out about 2 tablespoons of dough and flatten it slightly in your hand.

Place 1 teaspoon of Nutella in the center of the dough, followed by 1 teaspoon of marshmallow fluff on top of the Nutella. Press the edges gently to partially seal them, leaving a small gap to allow the marshmallow to puff out slightly while baking.

Take another 1 tablespoon of dough, flatten it, and place it on top, sealing the edges around the filling.

Repeat this process for the remaining dough, spacing the cookies about 2 inches apart on the baking sheets.

Bake in the preheated oven for 10-12 minutes, or until the cookies are slightly cracked on top.

The marshmallow fluff will spread into a smooth, gooey layer inside the cookie as it bakes.

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