Start with the pastry. Preheat the oven to 200C/400F. Line a large baking tray with baking parchment. Unroll one of the pastry rolls and place on the prepared tray. Sprinkle 2 tbsp of icing sugar (confectioners' sugar) on top, and smooth it all over the pastry with your hands. Place another sheet of baking parchment on top, then place a second baking tray on top (same size as the first). Place a small oven proof pan on top to weigh the top tin down.
Place in the oven and cook for 23-30 minutes until the pastry is a dark golden brown (check after 23 minutes, and if not golden, place back in the oven (you can take the pan off at this point) and continue to check every few minutes. Remove from the oven and repeat with the second pastry roll. I like to cook my pastry in two batches, as it cooks more evenly (and I haven't got 4 same-size trays!).
Once cooked, allow the pastry to cool, then cut into approx 11.5cm x 7.5cm rectangles. You should get 9 rectangles from each pastry sheet (18 altogether). This will make 6 mille feuille. You can cut the pastry into small pieces if you wish, just make sure you have a number that is divisable by 3, as you want 3 layers of pastry in each mille feuille.
Now make the chocolate creme patissiere (chocolate custard). Add the milk, cream and vanilla extract to a large pan. Heat until just before boiling point (don't let it boil).
Meanwhile, in a large bowl, mix the cornflour, egg yolks and caster sugar. Add a splash of the hot milk to the egg mixture and whisk in quickly. Continue to add, a splash at a time, stirring with the whisk afterwards, until all the milk is incorporated.
Give your pan a quick clean to remove any milk residue, then add the milk mixture back into the pan. Heat on a medium heat (don't let it boil), stirring with a whisk all the time until the custard is very thick. It will thicken quite suddenly after a few minutes. Turn off the heat and add in the butter and broken up chocolate. Allow to melt for a minute, then stir until the chocolate and butter melts completely.
Pour the custard into a bowl and allow to cool for 5 minutes, then place some clingfilm directly on top of the custard - so it's touching all of the top of the custard - and leave to cool completely (this prevents a skin forming on the custard). If you're making the custard in advance, you can place in the fridge once cooled until you're ready to use it.
Now it's time to assemble. Place 6 of the pastry slices on a serving tray/board. Spoon the chocolate custard into a piping bag with the end cut off (so it's got approx 1cm opening).
Pipe a layer of custard onto the pastry. Place another piece of pastry on top, pipe a second layer of custard on. Top with the remaining pastry - so you have 6 mille feuille altogether, each with 3 layers of pastry and 2 layers of custard.
Warm the Nutella up in the microwave for a few seconds, then drizzle or pipe the Nutella over the mille feuille. Sprinkle on the chopped hazelnuts, then top each mille feuille with 3 halves of chocolate caramel egg.
Serve immediately, or refrigerate and serve within a day.
Nutritional Information is per Chocolate Mille Feuille. This is a very high calorie (and large) dessert, so feel free to cut in half or even into thirds to reduce the calorie amount.