Preheat oven to 350 degrees; butter a 9-inch springform pan.
In a large bowl (preferably metal), whisk the egg whites and
salt until stiff. In another bowl, cream the butter and Nutella,
then add the rum, egg yolks, and ground hazelnuts. Fold in
melted chocolate.Add a blob of beaten egg whites to the
chocolate batter, and mix gently until well-combined. Fold in the remaining whites, one-third at a time, very gently but thorougly. Pour into springform and bake for 40 minutes. Test for doneness by inserting tester, which should come out mostly clean and let cool completely.Toast the hazelnuts in a dry skillet, shaking them around frequently. Do it for about 5 minutes, or until they are lightly browned, then let cool completely. If hazelnuts came with skins on, put them in a towel after toasting and rub around; this will remove most of the skins. You can use chopped hazelnuts. Chop chocolate, and add to sauce pan with cream and rum over medium-low heat. Once chocolate is melted and components are combined, whisk until mixture reaches desired thickness, then cool. Remove rim of cake pan and pour cooled ganache over, spreading lightly to create a smooth, shiny surface, and apply hazelnuts all over. Let the ganache settle and meld with the top of the cake resting at least an hour.