Preheat the oven to 190°C. Place margarine in a bowl, add sugar and beat together till light. Break eggs and add to the bowl and mix. Add yogurt and mix. Grate the rind of an orange and add to the mixture. Add orange juice and mix well.
Sift refined flour and baking soda into the bowl and mix. Add chocolate and mix well.Pour the batter into a rectangular silicon mould and level the surface.
Keep it in the preheated oven and bake. Check after 20 minutes if it is done. If it is not cooked completely, then bake for 5 minutes more. Removed from oven and cool. Remove from the mould, slice and serve.