Break the chocolate into smaller pieces and melt it in a double-boiler (you can use the microwave as well). Add the butter cut into pieces and mix well with a whisk to obtain a smooth texture. Pour the mixture into a bowl.
Add the sugar and mix well. Add the flour, and mix again vigorously, to avoid lumps. Crack the eggs into another bowl and beat them into an omelet. Add them to the chocolate mixture and mix well. You should have a thick, smooth, satiny batter.
Pour the batter into a 24/25 cm diameter cake tin (ideally in silicone), buttered and floured beforehand.
Lower the thermostat to 150°C/302°F. Bake for 30 mins.
Leave to cool for 10 minutes and unmold. You should obtain a cake with a light crust and a soft, melting and creamy center.