Preheat the oven to 180ºC/160ºC Fan/gas mark 4/350ºF. 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 Frangelico (or whatever you're using), egg yolks and ground hazelnuts.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.Pour 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.Toast the hazelnuts in a dry frying pan until the aroma wafts upwards and the nuts are golden-brown in parts: keep shaking the pan so that they don't burn on one side and stay too pallid on others. Transfer to a plate and let cool. This is imperative: if they go on the ganache while hot, it'll turn oily. (Believe me, I speak from experience.) If your hazelnuts have skins on then after toasting in the frying pan transfer them to a slightly dampened tea towel and rub them while they are still warm to remove the skins.In a heavy-bottomed saucepan, add the cream, liqueur or water and chopped chocolate, and heat gently. Once the chocolate's melted, take the pan off the heat and whisk until it reaches the right consistency to ice the top of the cake. Unmould the cooled cake carefully, leaving it on the base as it will be too difficult to get such a damp cake off in one piece.Ice the top with the chocolate icing, and dot thickly with the whole, toasted hazelnuts. If you have used Frangelico, put shot glasses on the table and serve it with the cake.