Line the loaf tin with greaseproof paper, across the bottom and up the two long sides. Divide the cream in two large bowls. Add the icing sugar to one, and whisk to soft peaks. Add 200 grams of Nutella to the other, and whip to soft peaks.
Assemble the cake: Layer one layer of plain, and one layer of Nutella cream in the bottom of the tin. Spoon a thick layer of Nutella over an ice cream wafer, and lay it face down on the cream. Repeat with 5 or so more wafers, until you have covered the layer of cream - leaving a small edge all the way around. Repeat three more layers, alternating the layers of plain and Nutella cream, until the loaf tin is full. Then cover with cling film and put in the freezer for 4 hours, or overnight. Save the remaining Nutella cream.
Take the ice box cake out of the freezer 30 minutes before you want to serve it. Dust the top with cocoa, using a wire rack to make a pattern. Pipe rosettes of Nutella cream around the edge, and sprinkle them with chopped hazelnuts. Use a hot knife to cut slices and serve.