Preheat the oven to 180°C conventional oven (350° Fahrenheit/Gas 4). Line two large shallow baking trays (approximately 33cm X 44cm in size) with baking paper.
To make the dough for the Chocolate and hazelnut cantucci, place the eggs and sugar into a large bowl. Use an electric mixer to beat the eggs and sugar together on medium speed until just combined. Add the Nutella, then a few cubes of butter at a time, beating until well combined and creamy.
Use a wooden spoon to stir in the flour and cocoa, one tablespoon at a time. Stir in the Italian baking powder to make a smooth dough. Lastly stir in the hazelnuts (or almonds) and add a pinch of salt – you may need to use your hands as the dough gets thicker.
Halve the mixture. Transfer the dough to one of the prepared baking trays and shape into 2 logs of 5cm in diameter (approximately 20-25cm in length), allowing 10cm minimum between each log. Wrap in plastic wrap and refrigerate for 15 minutes.
Note: The diameter of the cantucci can be increased but no more than 10cm. You will need to allow a longer cooking time for thicker cantucci.
Remove the logs from the fridge. Discard the plastic wrap (cling film). Bake for 20 minutes. Remove from the oven and allow to cool for 10 minutes.
Transfer the logs to a chopping board. Use a serrated knife to cut the logs into 1cm thick cantucci slices. Transfer the cantucci to two baking trays, flat-side down.
Bake for 4 minutes. Remove from the oven and turn each of the cantucci. Bake for a further 4 minutes.
Allow the Chocolate and hazelnut cantucci to cool completely before serving – buon appetito.