Preheat the oven to 180°C (350°F) and line a baking sheet with parchment paper.

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

In another bowl, cream the butter, granulated sugar, and brown sugar until fluffy.

Add the egg and vanilla extract, mixing well.

Stir in the Nutella until fully combined.

Gradually add the dry ingredients and mix until a soft dough forms.

Gently fold in the marshmallows.

Scoop dough onto the baking sheet, leaving space between cookies.

Bake for 10–12 minutes until edges are set but centers are soft.

Let cool for 5 minutes before serving.