Cut the cold butter into small cubes and place it in a large mixing bowl with flour, ground hazelnuts, sugar, egg, and baking powder. Mix until a smooth dough forms.

Divide the dough into two equal rolls, wrap each in plastic wrap, and chill in the refrigerator for about 30 minutes.

Remove the rolls from the refrigerator, unwrap, and slice into approximately 40 equal pieces. Arrange the pieces on a baking sheet.

Preheat the oven to 160°C (320°F) and bake for 12-15 minutes. Let the cookies cool completely after baking.

Melt the Nutella using a water bath. Take the bottom half of each cookie, add a dollop of Nutella, and top with another cookie. Gently press together.

Melt the dark chocolate coating in a water bath. Dip each filled cookie into the melted chocolate and sprinkle with chopped hazelnuts if desired.