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

In a bowl, whisk together the flour, cocoa powder, baking soda, and salt until well combined.

In another bowl, cream together the butter and Nutella using a metal whisk or a hand mixer. Alternatively, you can use a stand mixer with a paddle attachment.

Add the white and brown sugar and whisk well.

Add the egg and vanilla, and whisk until well combined.

Add the dry ingredients mixture and mix until no dry ingredients are left using a rubber spatula. Wrap the dough in plastic wrap and freeze for 30-60 minutes, or until the dough is no longer sticky and can be rolled into balls without difficulty.

Form 15 dough balls (approximately 35g each), and place them on the prepared baking sheets, spacing the balls about 2 inches apart.

Bake the cookies in two separate batches for 12-13 minutes or until the cookies are very puffy and have finished spreading.

Remove the cookies from the oven and allow them to cool on the baking sheet for 15-20 minutes. Transfer them onto a wire rack to cool completely.