Separate the whites from the yolks.
In a bowl, mix the egg yolks and caster sugar until the mixture becomes white and foamy.
Melt the chocolate in a double boiler or in microwave with 3 teaspoons of water.
Pour the melted chocolate into the sugar-egg mixture.
Add the flour, melted butter and four egg whites to the mixture and beat until stiff.
Butter a 20cm diameter cake pan and pour in the cake mix.
Bake at 170º C for 15 to 20 minutes. Cooking should be short to keep the smoothness of the cake. The point of the knife should come out candy.
Remove the cake from the oven and let cool before unmolding.