For the nut-chocolate cake, beat the egg whites with a pinch of salt until stiff. Beat egg yolks and powdered sugar until fluffy, then add whipped cream and mix again.
Stir grated walnuts and grated cooking chocolate, as well as rum and wholemeal flour into the yolk mixture. Add mineral water, fold in egg whites and pour into a greased, floured high cake pan.
Bake on the lowest setting at approx. 160 °C.