For the choux pastry:

1. Preheat the oven to 480°F (250°C) et line a large baking sheet with parchment paper. Place a large mixing bowl in the refrigerator.
2. In a medium saucepan, heat water, milk, salt and sugar on low heat. Remove the saucepan from the heat as soon as the liquid starts bubbling. Add the sifted flour and mix vigorously with a wooden spatula until no lumps remain.
3. Put the saucepan back on the heat and stir the dough until it detaches from the sides of the pan and makes a compact and smooth ball of dough. Transfer to the cold large mixing bowl and let cool for 5 minutes in the bowl placed at room temperature. Incorporate the eggs, one at a time, using a wooden spatula or a stand mixer equipped with a paddle attachment, stirring well after each addition.
4. Transfer the choux pastry in a large piping bag and pipe small amounts of dough, leaving enough space between each of them. Bake in the 480°F (250°C) preheated oven, then drop the oven temperature to 350°F (180°C) right away, and bake for 25 minutes (do not open the oven in the meantime!), or a little bit longer if needed, until the pastry choux become golden in color. Open the oven door and let the choux pastry 5 more minutes in the oven, then transfer to a cooling rack and let cool completely.

For the hazelnuts:

1. In a small saucepan, heat water, sugar and hazelnuts until you reach 240°F (118°C) (if you don’t have a thermometer, stop when the mixture become sticky and syrupy).
2. Transfer to a sheet of parchment paper and let cool.
3. Once the hazelnuts have cooled, transfer to a food processor and pulse a few times until crushed.

For the chocolate sauce:

1. Heat the cream until just simmering. Let sit for one minute, then pour over the chocolate, chopped finely (extremely important) and whisk until completely incorporated.

To assemble the profiteroles:

1. Slice each choux pastry in half, fill with vanilla ice cream and pour hot chocolate sauce on top.
2. Add the caramelized hazelnuts, and enjoy straight away!