- Preheat oven to 350˚F and roast hazelnuts for about 15 minutes.
- Rub to remove skin and allow nuts to cool.
- Coarsely chop nuts and two oz of chocolate into small chips and set aside.
- Grease a 9x5-inch loaf pan with ½ tablespoon of the margarine and dust lightly with potato flour.
- Place remaining chocolate, broken into small pieces, in top of double boiler.
- Cover and place over simmering water until melted.
- Gradually whisk in margarine until smooth and evenly mixed; set aside.
- Beat egg yolks, while adding sugar slowly until pale and thick (about 10 mins).
- Pour in cooled chocolate, beating until thoroughly combined; set aside.
- Beat the five egg whites at high speed until stiff.
- With a rubber spatula, fold half the nuts, the chocolate chips and a quarter of the egg whites into the chocolate mixture.
- Repeat with another quarter of the whites and the remaining nuts.
- Reverse the process, pouring chocolate over the remaining egg-whites.
- Gently fold the two mixtures together until uniform throughout.
- Pour the batter into the pan
- Bake in the center of the oven for about 55 minutes or until a cake tester comes out dry.
- Allow to cool.
- Loosen sides with a knife before removing cake.