Preheat the oven to 175° Celsius. In a large baking bowl over a bain-marie, gently dissolve the chocolate, butter, sugar and espresso. Stir occasionally. Meanwhile, in a separate baking bowl, mix the 3 whole eggs with the 3 egg yolks and fold into the chocolate mixture, it is recommended to preheat the egg mixture first with a little bit of chocolate mixture so that it does not curdle when mixed with the warm chocolate mixture.
Meanwhile, whip the egg whites until almost stiff, add the fine sugar and whip until very stiff. Carefully fold into the chocolate mixture with a whisk. Butter an ovenproof dish (preferably round with straight sides) and pour in the chocolate mixture.
Bake the cake for about 35 minutes until it has risen a little. When testing with a kitchen knife, the inside should still be wet. Cool for 1/2 hour, and place on the table garnished with orange slices and fine sugar.
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Tip: Feel free to use better chocolate – the more delicious the result!