Preheat the oven to 250°F (yes, this is right!).
In a large bowl, beat the butter and brown sugar for a few minutes, until light. Beat in the eggs and then the molasses. It may look curdled - that’s OK.
Put the raisins and currants into another bowl and shake a couple spoonfuls of the flour over them; toss to coat them well. In another bowl, stir together the remaining flour, cinnamon, ginger, nutmeg and salt. In a small bowl or measuring cup, stir together the coffee and baking soda.
Add about a third of the dry ingredients to the butter mixture, then half the coffee, another third of the dry ingredients, the remaining coffee and the remaining dry ingredients, mixing just until smooth. Stir in the dried fruit mixture.
Divide the batter between two parchment-lined 9x5-inch loaf pans, smoothing the top. Bake until a wooden pick or skewer inserted in the middle of each cake comes out clean, 2 1/2 to 3 hours. The cakes will sink slightly in the middle - that’s OK.
Cool for about 10 minutes and then lift the cakes out of the pans to cool on wire racks. If you can, cool and wrap them well, and store at room temperature for a couple days before you slice them. Makes 2 fruitcakes.