Preheat your oven to 350°F (180°C).
Rinse the oranges under hot water and let them dry off, then grate the zest from one and press out the juice from both.
In the bowl of your stand mixer fitted with the paddle attachment beat butter, sugar and a pinch of salt together until creamy (at least 8 minutes), then stir in one egg at time.
Mix flour and baking powder together and add to the mixture together with 5 tbsp orange juice. Mix until well incorporated.
Divide the batter evenly between two bowls. To one bowl add the orange zest and give it a good stir. To the other bowl add cocoa powder, chocolate, and 5 tbsp orange juice.
Grease a loaf pan (8 1/2 x 4 1/2 x 2 1/2) and start with a layer of half of the light batter. Add a layer of dark batter, a layer of light batter and a layer of dark batter. Use a fork and mix the layers spirally two times.
Bake the cake for 50 minutes (test with a skewer). After ten minutes cut the cake lengthwise with a knife.
Let the cake rest in the loaf pan for ten minutes and then transfer it to a cooling rack. When the cake is completely cooled down, mix icing sugar with lemon juice until it has a good consistency and decorate your cake.