The mix of sugar, eggs and vanilla sugar are whisked to get a frothy mass. Then, with constant stirring add the milk, cooking oil and lemon zest. Finally, add gradually the flour and the baking powder, whisking at slow speed. Add chocolate to one third of the mixture. Pass the mixture in several places with a fork to decorate the cake. Bake for 1 hour at a temperature of 180 ° C. Decorate it as desired.