Preheat the oven to 180°C / 350°F / Gas mark 4.
In a large bowl, whisk the egg whites and salt until stiff. In a separate bowl, beat the butter and nutella together, and then add the run or water, egg yolks and ground hazelnuts.
Melt the chocolate in a bowl over simmering water or in the microwave then add it is once cooled, add a large dollop of egg white, and mix well before gently folding the rest of the eggs whites in a third at a time.
Pour into a 23cm 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.
In a deep frying pan, toast the hazelnuts until they become golden brown.
Shake the pan while they are toasting.
Transfer to a plate and let cool.
If the hazelnuts have skins on, transfer them to a slightly dampened tea towel and rub them while they are still warm to remove the skins.
For the icing, use a saucepan and add the cream, liqueur or water and chopped chocolate, and heat gently.
Once the chocolate has melted, take the pan off the heat and whisk until it reaches icing consistency.
Unmould the cooled cake carefully, leaving it on the base.
Ice the top with the chocolate icing, and dot with the whole, toasted hazelnuts.