In a small, shallow bowl, sprinkle the yeast and 1/2 teaspoon of sugar over 1/2 cup of lukewarm milk.
Let the mixture stand for 2 or 3 minutes, then stir to dissolve the yeast completely.
Set the bowl aside in a warm, draft free place (such as an unlighted oven) for 5 to 8 minutes, or until the mixture doubled in volume.
In a deep mixing bowl, combine the remaining sugar with salt and flour.
Make a well in the center and pour in the yeast mixture.
Add the remaining milk, drop in the eggs and, with a large spoon stir the flour into the liquid ingredients.
Continue to stir until fairly stiff dough is formed.
Knead the dough by pulling it with your hands (as though are pulling taffy) for about 10 minutes, or until it is smooth and elastic.
Drape the bowl loosely with a kitchen towel and set it aside in the warm, draft free place for 1 hour, or until the dough doubles in bulk.
Then punch it down and knead in 3/4 pound of the softened butter, a tablespoon or so at a time.
Knead in the currants and sultana raisins.
With a pastry brush, coat the inside of a 12-cup mold with the remaining 2 tablespoons of softened butter.
Pat the dough into the mold; it should come about halfway up the sides.
Cover the mold loosely with a kitchen towel and set aside in a warm, draft free place for about 1 hour, or until the dough has doubled in volume and has risen almost to the top of the pan.
Preheat the oven to 400° F.
Bake the cake in the center of the oven for 10 minutes, then lower the heat to 350° F and bake another 35 minutes.
Remove from the oven, drape a kitchen towel loosely over the top of the mold and let it rest for 5 minutes.
Run a knife around the inside of the mold and turn the cake out onto a serving dish.
Then carefully turn the cake over on the plate.
Combine the sugar and water in a small saucepan and, stirring constantly, bring to a boil over high heat.
Cook briskly for 5 minutes, undisturbed, until the mixture forms a syrup thick enough to coat a spoon lightly.
Pour the syrup into a bowl, then stir in the rum.
With a wooden spoon, mix together the sugar, water and lemon juice.
Slowly spoon the warm syrup over the warm cake.
With a large spoon, baste the cake every 10 minutes or so with the syrup collecting around the cake.
When all the syrup has been absorbed, you may ice the top of the cake with
the
white icing just before serving, if desired.