Yield, three loaves. Can be doubled but most mixers can't handle that much dough.
The night before (or at least one hour): combine raisins and currants in a non-reactive two-quart bowl (glass works well). Cover with rum, adding more if need be. Stir, cover with plastic wrap and set aside.
The next day: scald milk and remove from heat. Add chilled butter. Stir until the butter is completely melted. The milk-butter mixture should be the appropriate temperature for yeast. Pour into large mixing bowl and add sugar and yeast; stir. Let sit for 10 minutes, and check that yeast is foaming. This is the sponge.
Drain dried fruits and reserve rum (if you used water to re-hydrate, don't bother reserving the liquid). Dredge in an additional 1/4 to 1/2 cup of flour. Add two tablespoons of the reserved rum to the sponge. Lightly beat the eggs and add to the sponge.
Add six cups flour and cardamom to the sponge. Add raisins, nuts and currants. Add more flour if dough is too wet and sticky to work. Remove rings and knead until smooth and elastic (8 to 10 minutes by hand, or 2 minutes after dough clumps around dough hook in a stand mixer). Place in oiled bowl, turn, cover with a clean tea towel and let double in bulk, about one hour. The dough will be fairly sticky; you may want to use a mixer if you think it's too sticky.
Punch down dough. (This is fun.) Divide dough in three parts (a scale is helpful, dough for each loaf should weigh about 650 to 700 grams). Knead, shape into an oval, and fold over, not quite in half. Place on ungreased cookie sheet. Repeat for remaining dough. Three loaves should fit on one large sheet.
Allow dough to rise for another hour.
Preheat oven to 350 degrees, and bake for 45 minutes to an hour. Bread will sound hollow when tapped. Cool on racks.
When ready to eat, melt butter, brush on loaf and sprinkle a liberal amount (nearly 1/4 inch) of powdered sugar. If you are giving these as gifts, skip this step and wrap in plastic instead. A nice touch is a shaker of powdered sugar (unscrew top, fill with sugar, place a piece of plastic wrap over opening, replace top) tied on to the top of the loaf. Your lucky recipients can supply their own butter.
Notes: You aren't limited to the fruits I suggest, or the ratios I suggest. Other recipes include walnuts, Citron, fruitcake mix and lemon rind. I don't include them because I hate walnuts, citron and fruitcake mix. The cardamom is optional too. (Store that in the freezer when you are finished.)
If you want a lighter loaf, you can increase the sugar by two to three tablespoons (one of my "mistakes") or leave out the eggs (another "mistake"). You can also add some salt to the dough -- no more than one teaspoon.