1. Heat the oven to 392°F (200 °C).
2. Add the butter to the chocolate and melt it in the microwave or in a steam bath.
3. Stir well until you get a homogeneous mixture, then add the powdered sugar and vanilla and stir again. Leave it aside for 5 min. to cool.
4. Break the egg into a bowl and stir well.
5. Begin to add the egg to the chocolate a little bit at a time, while stirring nonstop.
6. Add the flour to the mixture and stir until homogeneous.
7. Cover a well-buttered bowl with flour, get rid of any remaining flour by tapping the bowl on the work surface.
8. Pour the mixture into the bowl and put it in the oven for 12 min.
9. Once it's ready, wait for it to cool for 1 min., then invert onto a plate and remove the bowl.
10. Finish by sprinkling the cake with powdered sugar if desired.