Mix the dried cherries, prunes, raisins, candied orange peel, and currants in a jar. Pour 4 tablespoons of rum over the fruit. If they aren't covered with rum, or if your fruits are on the dry side, add the additional tablespoon. Cover the jar, shake it and let the fruit sit for at least 24 hours or up to one week. (Although you can let it sit for longer.)
To make the cake, preheat the oven to 300ºF (150ºC.) Butter an 8 1/2- to 9-inch (23cm) loaf pan. For extra insurance, you can line the bottom of the pan with parchment paper.
Whisk together the flour, baking powder, salt, and spices in a small bowl. Set aside.
In a stand mixer fitted with the paddle attachment, or by hand in a large bowl using a spatula, beat the butter and brown sugar on high speed for 3 minutes, until light and fluffy. On low speed, add the eggs one at a time, stopping the mixer after adding each egg to scrape down the sides.
Stir in the vanilla, aromatic bitters (if using) and molasses. The batter may appear a bit curdled. If so, beat it on high speed for a few minutes which will aerate it and bring it back together. Remove the bowl from the mixer and stir in the flour mixture by hand, mixing it only until the dry ingredients are abotu three-quarters combined. Fold in the rum-soaked fruits (they should have soaked up most or all of the rum; if there is a small amount of rum in the jar, like a tablespoon or less, add that to the batter, too) as well as the walnuts and stir just until combined but do not overmix.
Scrape the batter into the prepared loaf pan, smooth the top, and bake until a toothpick inserted into the center comes out clean, about 1 hour, 15 minutes. (The original recipe said 1 hour, 45 minutes, but mine was done sooner, which may be because my loaf pan is narrower and higher than a typical loaf pan.) Remove from the oven and use a toothpick to poke about 50 holes in the top. Pour the 1/3 cup of rum all over the cake. Let the cake cool completely.