Preheat the oven to 200°C (390°F) and line one large baking tray with parchment paper.
On a clean work surface or cutting board dusted lightly with cake flour, remove the puff pastry from its packaging. Use a rolling pin to lightly roll out the pastry to ensure it is flat.
Use a spoon to scoop the Nutella onto the pastry, then use an offset spatula to spread the Nutella evenly over the entire surface of the pastry.
Carefully place the second layer of pastry on top of the Nutella layer, ensuring that it lines up perfectly with the bottom layer.
Use a ruler to measure the cut lines, on one end space the lines 10cm (4 inches) apart, and on the opposite end space them 5cm (2 inches) apart. Use a pizza cutter or sharp knife to cut diagonally across the pastry creating large long triangles.
Starting at the wide end of the triangle, roll the croissant tightly upwards using your fingertips, once the croissant is rolled, bring the tips of the croissant towards each other to create a crescent croissant shape. Place the rolled croissants onto a lined baking sheet, with the tip of the triangle at the bottom of the croissant.
Lightly beat the egg in a small bowl and use a pastry brush to brush the surface of the croissants with the egg wash.
Sprinkle the chopped hazelnuts over the top of the croissants.
Bake the croissants in a preheated oven for 40 minutes or until golden brown. Once baked, dust with icing sugar, serve and enjoy!