Mix butter and sugar until light and creamy.
Add egg yolk one by one, while mixing.
Fold in flour.
Spread mass on baking tray with baking paper.
Whisk egg whites, sugar and salt until stiff.
Fold in nuts.
Spread nut mixture evenly over the dough.
Bake in preheated oven at 180°C for 30 minutes.
Leave to cool.
Decorate with melted chocolate if desired.