Pour the remaining milk over the sponge. Add the egg yolks, sugar and salt and beat until the ingredients are well blended. Add 1 cup flour and beat well. Blend in the butter. Add more flour gradually to make a smooth dough, or until 10 to 11 cups have been added.
Some flours absorb more liquid than others.
Knead in the sultanas, almonds, and citron, along with the lemon rind which should be mixed with the rum. Dust the top lightly with flour and let it rise in a warm place about 45 minutes. Punch it down and let stand for 20 minutes. Divide the dough in half and knead the pieces until smooth. Let them stand for 10 minutes longer.