Melt chocolate au bain-marie.
Mix flour and nuts.
Whisk butter and icing sugar until creamy and light.
Stir in yolks one by one.
Mix in melted chocolate.
Beat egg whites, sugar and salt in a separate bowl until stiff.
Alternately fold the egg whites and flour-nut mixture into the butter mixture.
Spread the mixture on a baking tray with baking paper.
Bake in preheated oven at 200° C for about 20 minutes.
In the meantime mix the rum and icing sugar.
Spread the glaze on the still hot cake.
Let to dry briefly and cut into squares while still warm.
Store chilled in a box.