Preheat oven to 180 C / Gas mark 4. Grease a large baking tray.
In a large bowl, cream together the 400g sugar and margarine until smooth. Beat in eggs one at a time, mixing well after each one. Combine the cake meal, potato starch and salt; stir into the creamed mixture. Mix in walnuts and chocolate chips. The mixture will be heavy. Form into 2 long oval logs. Place onto the prepared baking tray. Sprinkle the top with a mixture of 2 teaspoons sugar and cinnamon.
Bake for 50 to 55 minutes in the preheated oven, until a skewer inserted into the centre comes out clean.
Slice the logs into thick slices; set aside to cool.
If you prefer, you can arrange the sliced biscuits on the baking tray, and put back in a very low oven to dry out.