Chop the chocolate, melt with butter over a hot water bath. Remove from the water bath and stir well.
Stir in 5 egg yolks with a whisk. Sift the flour on top and stir in briefly with a pinch of salt. Beat the egg white and a pinch of salt until stiff, then gradually pour in the sugar and beat for another 2 minutes. Fold the egg whites under the chocolate mass.
Spread in a springform pan lined with baking paper. Bake in the oven at 160 degrees convection or 180 degrees top / bottom heat on the second rack from the bottom for about 50 minutes.