Prepare the puff pastry by cutting it into even rectangles, then brush each piece with egg wash and lightly sprinkle over the brown sugar.
Bake the pastry in a preheated oven at 180°C (350°F) for around 10 minutes until golden, puffed and crisp.
While the pastry bakes, chop the hazelnuts and warm the chocolate hazelnut spread in the microwave until smooth and pourable. Transfer it into a piping bag or keep it ready to spoon.
Whip the cream by adding the double cream, icing sugar and vanilla bean paste to a bowl, then whisk until softly whipped. The cream should hold its shape but still be smooth. Transfer the cream into a piping bag and cut the tip at a slight angle right before piping.
Assemble the cream puffs by allowing the pastry to cool slightly, then slicing each piece in half horizontally. Add a layer of chocolate spread to the base, sprinkle over chopped hazelnuts, then add the whipped cream. Place the top layer back on and finish with extra chocolate drizzle and hazelnuts.