Step 1: In a large bowl, combine the mixed dried fruits with your chosen soaking liquid (brandy, rum, or orange juice). Cover and let soak overnight at room temperature, or for at least 6-8 hours, stirring occasionally.
Step 2: Preheat your oven to 300°F (150°C). Grease and line a 9-inch round cake pan with parchment paper, extending the paper about 2 inches above the rim to prevent the cake from overflowing and to protect the edges during the long bake.
Step 3: In a large mixing bowl, cream together the softened unsalted butter and brown sugar until light and fluffy. This should take 3-5 minutes with an electric mixer.
Step 4: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract and molasses (or treacle).
Step 5: In a separate bowl, whisk together the all-purpose flour, baking powder, ground cinnamon, nutmeg, cloves, and mixed spice. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 6: Drain any excess liquid from the soaked fruits (if any) and fold the fruits, along with the orange zest and chopped nuts, into the cake batter until evenly distributed.
Step 7: Pour the batter into the prepared cake pan, smoothing the top with a spatula. Bake for 2 hours 30 minutes to 3 hours, or until a skewer inserted into the center comes out clean. If the top darkens too quickly, cover loosely with foil.
Step 8: Remove the cake from the oven and let it cool in the pan for 15-20 minutes before inverting it onto a wire rack to cool completely. Once cool, if desired, brush the cake lightly with an additional tablespoon or two of your chosen soaking liquid for added moisture and flavor (especially if using alcohol).
Step 9: For best results, allow the fruit cake to age for a few days or even weeks. Wrap it tightly in parchment paper, then foil, and store in an airtight container in a cool, dark place. Periodically 'feed' it with a little more liquid if you like, especially if using alcohol.