1.Preheat oven to 150°C. Grease a deep 20cm round cake pan and line base and side with baking paper.
2.Stir chocolate, butter, sugar and water in a medium saucepan over low heat until smooth. Transfer to a medium bowl and allow to cool for 15 minutes.
3.Stir sifted flours, ground hazelnuts, eggs and liqueur into chocolate mixture. Pour mixture into pan. Bake cake about 1 hour 35 minutes. Stand in pan 5 minutes. Turn top-side up onto a wire rack to cool.
4.Meanwhile, to make fudge frosting, stir butter, brown sugar and water in a small saucepan over heat, without boiling, until sugar dissolves. Remove from heat and stir in liqueur. Sift icing sugar and cocoa into a small bowl; gradually stir in hot butter mixture until smooth. Cover and refrigerate about 15 minutes or until frosting thickens. Beat frosting with a wooden spoon until spreadable. Spread cooled cake with frosting.