Dissolve yeast in 120ml of warm milk and set aside in a warm place for 10 minutes. Soak the sultanas in the rum.
Sift 1 cup (125g) flour into a bowl. Add the remaining milk and mix well. Add the yeast mixture, mix well, cover and set aside in a warm place for 30 minutes.
Beat the egg yolks in a separate bowl with the sugar until pale. Beat in the vanilla essence and the rum which was used to soak the raisins. In another bowl, beat the egg whites with a pinch of salt until fluffy.
After 30 minutes, combine the dough and egg yolks, then fold in the egg whites. Gradually add the remaining flour until you have a wet but workable dough, turn out onto a floured surface and knead for 10 minutes, working in all the remaining flour.
When the dough feels elastic, knead in the butter. Transfer the dough into a lightly greased bowl, cover with cling wrap and place in a warm place for 1 1/2 hours to rise. Punch down the dough and knead for a couple of minutes.
Sprinkle a little flour over the sultanas to coat them. Sprinkle the sultanas, nuts and mixed peel around the dough and knead them in.
Divide the dough in half and place into two large tins which have been lined with parchment paper. Cover with a tea towel and let rise until the dough reaches the rim of the tin. Meanwhile, preheat the oven to 180 degrees C.
Place the tins in the oven and bake for 45-60 minutes, depending on tin size. Remove from tins, remove the parchment paper and let cool. When cool, ice with plain white icing.