Preheat oven to 300° F. Butter and flour a 10-inch cake pan.
Whisk together dry ingredients in a medium bowl, then rub the tablespoon of butter into them until very fine. In another medium bowl, combine the raisins, currants, and caraway seeds with a tablespoon of the dry mix, breaking up any clumps of fruit with your fingers. Stir the fruit and seeds into the rest of the dry mix.
If you wear rings, remove them now! (Or plan on scraping the dough from the crannies of your jewelry later.) Pour the buttermilk into the dry mix and stir with a wooden spoon until a dough begins to form. When it does, knead the dough into a ball (dusting with flour if you need) until cohesive, about 2 minutes.
Place the dough into the pan and bake 60 to 70 minutes, until the crust begins to turn pale gold and makes a hollow sound when you tap it.