Preheat the oven to 145 degrees Celsius / 295 Fahrenheit.
Make the nut paste by sieving the almond and hazelnut flours and icing sugar to remove any lumps. Then blend them all together until they are a very fine consistency and thoroughly combined. Stir in the cocoa powder and put the mixture into a large mixing bowl.
Mix the first lot of egg white with the ground nuts/powdered sugar mixture until it is fully incorporated.
Prepare the Italian meringue. Place the water and granulated sugar into a saucepan. Gently warm the water and stir to dissolve the sugar. Turn up the heat, stop stirring and place the sugar thermometer into the mixture.
Once the temperature reaches 115 degrees Celsius/240 degrees, whisk the other egg whites in a separate large bowl until they form soft peaks. Then slowly start to pour the sugar syrup into the eggs, whilst continuing to whisk.
The Italian meringue will turn a smooth, glossy and opaque white. Continue to whisk once all of the sugar syrup has been added to bring down the heat until you can touch the side of the bowl and it feels warm but not hot.
Mix the Italian meringue into the nut paste in three stages. Firmly mix in the first third of the Italian meringue with a metal or wooden spoon to loosen the paste. Fold in the remaining two-thirds one at a time using the softer spoon or spatula, being very gentle with the final third to not overmix the batter and make it too slack.
Pipe the macaron shells. Transfer the macaron batter to a piping bag, relax it slightly with your hands and then cut the tip off. Pip the macaron shells onto a lined baking tray, leaving a 2cm gap in between each macaron.
Pick up the tray and firmly smack it down onto a work surface twice to settle the Italian macaron batter and help prevent air bubbles.
Bake the chocolate hazelnut macaron shells straight away for 17 minutes, then remove them from the oven and allow them to cool on the tray.
Make the Ferrero Rocher macaron filling. Heat the cream in a saucepan until it reaches steaming point. Break the chocolate into small pieces and place into a bowl with the Nutella. Pour the hot cream over the top and stir to form a smooth and rich chocolate hazelnut ganache. Put the Ferrero Rocher filling into a piping bag and chill in the fridge.
Make the topping. Melt the chocolate in a bowl over a saucepan of steaming water and chop or blend the hazelnuts into small pieces.
Mix the chopped hazelnuts into the melted chocolate and dip half of the macaron shells into it to coat the domed outside of the shell, before placing them on a a tray in the fridge to harden.
Finish your Ferrero Rocher macarons. Pipe the Ferrero Rocher macaron filling onto the uncoated macaron shells, and sandwich together with one that has been coated in the chocolate and chopped hazelnuts.