Break the chocolate in small pieces and melt it a bain-marie (place it into a bowl suspended over a saucepan of simmering water, stir occasionally) or in microwave until smooth, shiny and glossy. While the chocolate is melting, in a bowl whip egg whites with a pinch of salt until forming stiff and firm peaks.

Melt the butter into the hot chocolate, stir well and and let it cool.

Preheat the oven at 180 °C.

In a separate bowl beat the sugar and yolks until creamy and smooth, then sift the flour straight into the bowl, and mix again (all lumps have to be gone). Add the chocolate and butter mixture and beat it until homogeneous and fluffy.

Slowly incorporate the white eggs with a mixing - spoon going up to down into it. Butter and flour a 22 cm diameter cake pan, pour the dough into it and cook for 25 - 30 min. When completely cool dust with icing sugar.