Put Unsalted Butter (125 gram) and Brown Sugar (1/2 cup) into a large mixing bowl. Beat them together with an electric hand mixer.
Add in Egg (1) and Nutella® (1/2 cup). Beat all of that together.
Sift All-Purpose Flour (1 3/4 cup) and Baking Powder (1 teaspoon) over the chocolate mixture.
Add in Dark Chocolate Chips (3/4 cup). Gently mix everything together with a wooden spoon.
Take tablespoonfuls of the mixture and roll them in your hands to make a little ball. Put them on a tray lined with baking paper.
Use a fork to gently press the balls. Put the trays into an oven preheated to 180 degrees C (350 degrees F) for about 12 to 15 minutes.
Take the cookies out of the oven and let them cool for ten to fifteen minutes until they firm up a bit. Then take them off the tray and serve.