Grease and line with parchment paper a rectangular mold (mine is 22 x 10 cm).
Melt the chocolate and butter in the microwave in 30 second intervals.
Add the sugar and whisk.
Separate the egg whites from the yolks. Add yolks to the chocolate and mix.
Beat the egg whites to stiff peaks in the mixer.
Carefully fold the whites into the chocolate mixture until they are fully incorporated. Do not overmix.
Spread this mixture in the pan you prepared, as evenly as possible. Cover with cling wrap and freeze until solid.
For the chocolate praline:
Preheat the oven to 200 º C.
On a tray, toast the hazelnuts in the oven for about 5 to 10 minutes.
Bring the water and sugar to a boil, reaching around 110 º C (about 5 minutes).
Add the hazelnuts and mix - the sugar will crystalize due to the difference in temperature.
Continue to mix until the sugar begins to melt and caramelize. Keep mixing until all the sugar has caramelized.
Remove the hazelnuts from the heat and transfer to a tray to cool down.
Set aside about 50 grams for the decoration. Place the remaining hazelnuts in the food processor.
Blend on high speed. Eventually, you will have a creamy paste. Set aside.
For the decoration:
Unmold the mousse into a serving plate. I recommend you place the cake pan on a tray with warm water for a few seconds, to help release it from the pan, then invert onto to the plate.
Spread the praline carefully on top of the mousse.
Coarsely chop the hazelnuts you set aside earlier. Decorate the mousse with these hazelnuts.