Preheat the oven to 180°. Melt the chocolate into pieces in a water bath or in the microwave.
Cut the butter and melt it.
In a bowl, break the eggs and put the sugar. Mix well until the mixture becomes slightly foamy.
Add the butter, and stir.
Add the melted chocolate, then mix it.
Incorporate the flour little by little.
Butter a pan and pour the dough into it.
Bake for 20 minutes with regular monitoring.