Preheat oven to 180°C.
Chop 200 gr. of chocolate and melt with butter in a water bath.
Mix eggs with sugar. Add flour and mix.
Incorporate melted chocolate to this preparation.
Butter mould with oil and pour the mixture.
Chop the rest of the chocolate (100 gr.) and put on top of the cake. With the weigh, the piece of chocolate will sink a bit into the preparation.
Bake for 30 minute and let it cool before unmolding.