Preheat the oven to 170 ºC hot air.
Cream butter with 2/3 of the sugar. Beat in the egg yolks one at a time, add salt and lemon zest.
Sift flour with baking powder and mix with raisins, candied fruit, chopped nuts and chopped chocolate and add to the mixture. Stir in the milk and rum as well.
Finally, beat the egg whites with the last third of sugar and a pinch of salt until stiff. Carefully fold in the beaten egg whites.
Pour into a greased and flour-dusted loaf pan and bake for about 1 hour.