Preheat the oven to 190°C.
Grease a cake tin. Cream sugar and butter together till light and fluffy.
Break the eggs into the mixture, add yogurt and mix well. Add orange juice and orange rind and mix. Sieve flour and soda bicarbonate into it and mix well. Add grated chocolate and mix.
Pour the batter into the prepared tin and bake in the preheated oven at 190°C for twenty five minutes.
Remove onto a wire rack to cool. Slice and serve.