Prepare the Nutella Filling: Line a tray with parchment paper. Using a teaspoon, portion 5 dollops of Nutella (about 1 tsp each) onto the tray. Freeze for 30 minutes or until firm.

Melt the butter: Melt the butter in a pan or in a microwave safe bowl until it’s fully melted.

Cream Butter and Sugars: Add granulated and brown sugars to the butter. Whisk vigorously for 3-4 minutes until creamy. Ensure the mixture isn’t hot before proceeding.

Incorporate Egg and Leaveners: Mix in 1 egg yolk (or half a whole egg), baking soda, and baking powder until smooth. The mixture should become creamier.

Add Flour: Switch to a spatula. Gradually fold in all-purpose flour just until no dry streaks remain. Do not overmix.

Shape and Fill the Cookies: Divide dough into 5 equal portions. Press a thumbprint into each making a dent into the cookie dough, insert a frozen Nutella dollop, and seal tightly. Roll gently to smooth cracks.

Chill Before Baking: Press chocolate chips onto the exterior. Freeze shaped cookies on a tray for 1 hour (or refrigerate overnight) to prevent spreading.

Bake: Preheat oven to 190°C (375°F). Space cookies 3 inches apart on a parchment-lined tray. Bake for 13 minutes (soft) or 15 minutes (crisp).

Cool and Decorate: Let cookies rest on the tray for 2 minutes, then transfer to a rack. Once cool, drizzle with Nutella and top with halved Ferrero Rocher.