Preheat oven to 180°C/160°C Fan. In a large bowl, whisk the egg whites and salt until stiff but not dry. In a separate bowl, beat the butter and nutella together, and then add the water, egg yolks and ground hazelnuts. I held back about 5g hazelnut for topping.

Fold in the cooled, melted chocolate, then lighten the mixture with a large dollop of egg white, which you can beat in as roughly as you want, before gently folding the rest of them in a third at a time.

Por into a 23cm/9 inch round greased and lined springform tin and cook for 40 minutes or until the cake's beginning to come away at the sides, then let cool on a rack.

With a small amount of barely boiling water in a pan, put an ovenproof bowl on top and ensure it doesn't touch the water. Melt the chocolate and butter then remove from the heat and add sieved icing sugar and double cream. Mix well and allow to cool briefly before spreading evenly on top of cake. Sprinkle with chopped hazelnut. Allow to set and then dig in.