Delicious cake to use up egg whites.
Grease and flour bundt pan (or another baking tin)
Chop the chocolate into pieces.
Melt with the butter. (over bain-marie or in oven at 80° C)
Beat the egg whites, sugar and salt in a clean (fat-free) bowl until stiff.
Sift the flour over it and fold in.
Stir in the melted butter-chocolate-mixture.
Fill in prepared bundt pan.
Bake in preheated oven at 180° C for about 35 minutes. (Do the toothpick test.)
After about 10 minutes, remove from the tin and leave to cool.
Sprinkle with icing sugar.