Heat the butter in a pan in a water bath and add the crushed chocolate to it. Stir until melted and homogenized. Lightly toast the nuts in a dry pan or in the oven and mix 2-3 tablespoons of powdered sugar with the coffee.
Separately, beat the egg yolks with the remaining sugar to whitening and add the nuts, sugar-coffee mixture, melted chocolate and the rum. Finally, mix in the already whisked to snow egg whites with a pinch of salt. Put the cake mixture into a form with a diameter of 10 1/4″ (26 cm), covered with baking paper.
Put the cake to bake in a heated 338°F (170 °C) oven for about 40 minutes. Allow it to cool for one night, before inverting it onto a tray and serving it. You can add the chocolate that's been melted with a few tablespoons of rum.