Preheat oven to 180 C / Gas 4, Grease and line a 23cm springform tin.
In a large bowl whisk egg white and salt until stiff.
In a seperate bowl beat butter and Nutella together, then add Tia Maria, egg yolks and ground hazelnuts. Gently fold in the melted chocolate, then lighten with a spoonful of egg whites. Then gently fold in the rest of the egg whites.
Pour mixture into prepered tin and bake in the oven for 40mins or till it comes away from the sides of the tin.
For the icing toast hazelnuts in a dry frying pan until golden.
In a saucepan add cream, Tia Maria and chocolate, once chocolate has melted and combined with cream. Remove from heat and allow to cool to a consistancy whereby it can be spread over the top of the cake and not run down the sides.