Mix the softened butter in a bowl until creamy and mix with sugar and egg yolk, add the melted chocolate, sieve the flour into the bowl and stir everything together until smooth. Beat the egg whites until stiff and carefully fold in the egg whites,
Brush a round springform pan with a little butter, add the dough and smooth it out. Bake at 180 ° C (preheated, top / bottom heat) on the bottom rack for about 45 minutes.
After baking, let it cool down a bit and sift icing sugar over the cake.