Roast the hazelnuts for a few minutes, then remove the skins by rubbing in a clean cloth and chop roughly.
Melt the chocolate in a basin over a pan of hot water. As soon as it is melted, remove from the heat. Melt the butter.
Beat the egg with the sugar, coffee and vanilla essence. Stir in the butter and the chocolate, then the hazelnuts and glacé cherries and finally the potatoes and crushed biscuits. Press into a small, greased loaf tin and put in the refrigerator for at least 2 hours. Turn out and serve in thin slices.