Preheat oven to 180°C.
Break eggs in a bowl. Add castor sugar and beat with an electric beater till light and fluffy.
Cream butter and icing sugar in another bowl with an electric beater till light.
Add beaten egg mixture, little by little, and cream well with electric beater. Scrape the sides of the bowl and beat further.
Place the chocolate in the microwave for 10 seconds to melt it. Remove from heat and mix well till it reaches room temperature.
Combine walnuts and refined flour and mix well.
Add the melted chocolate in the butter-sugar mixture and mix well.
Add flour-nut mixture and mix well. Pour the batter into a silicon mould filling it ¾, place on a baking tray and put in a preheated oven. Bake for 30-40 minutes.
Remove from heat, cool and demould. Cut into pieces and serve warm.