The Day Before Baking: Combine the dried fruit in a large mixing bowl. Add the dark rum, cover, and allow the mixture to soak at room temperature for a minimum of 12 hours, or ideally 24 hours prior to preparing the fruit cake batter.
Prepare the Fruitcake: Preheat the oven to 300°F (150°C) with a rack in the center position. Set aside two 8×4-inch loaf pans, line or grease the pans prior to adding the batter.
In a medium mixing bowl, whisk together the all purpose flour, baking powder, spices, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream the softened butter and light brown sugar together over medium-high speed for 3 minutes, or until light and fluffy. Reduce speed to medium and add the eggs one at a time, beating just until each egg is incorporated. The mixture will look slightly broken, that’s ok.
Over low speed, slowly add the flour mixture until just absorbed. Remove the bowl from the stand mixer. Add the lemon zest, orange zest, fresh orange juice, grated apple, slivered almonds, diced candied ginger, soaked dried fruit mixture (along with any liquid if it hasn’t already been absorbed), and chocoate (if using).
Stir mixture with a large spatula, scraping the edges and bottom of the bowl several times, until all of the ingredients are evenly incorporated. It is a very thick, fruit heavy mixture.
Transfer and divide the batter evenly among the loaf pans. Use an offset spatula to smooth the batter into an even layer. Set the loaf pans on the center rack, several inches apart from one another. Bake – rotating the pans halfway – for 75 to 90 minutes or until the batter is set and the tops are golden brown. The edges of the cake may also start to pull away slightly from the edges of the pan. Remove from the oven and place on a rack to cool completely before removing from the pans.
Storing and Aging: Once the cake has cooled, carefully remove the cakes from the loaf pans and set upright. Soak two large pieces of cheesecloth in medium sherry or triple sec (*they should be lightly damp, not soaking wet when you wrap the cakes, so you may need to squeeze out any excess liquor). Wrap each cake tightly. Cover loafs with wax paper and foil before transferring to a large Ziploc bag. Store in the refrigerator for up to 6 to 8 weeks (it can often last even longer). You can serve this cake right after it has cooled, but the flavors improve greatly with time (I recommend several days or up to a week, at minimum). Unwrap and re-soak the cheesecloth once a week.
For Garnishing: If desired, you can top the fruitcake with a light apricot glaze before serving. Note: I don't recommend glazing the cake if you wish to continue to wrap and age the fruitcake.
To Prepare the Glaze: Combine the apricot preserves and water in a small saucepan. Bring to a light simmer, stirring until the glaze is shiny and thin. If it is too thin for glazing, reduce to desired consistency. If it becomes too thick, add a teaspoon of water. Brush the tops of the fruitcakes with apricot glaze.
Serving Notes: Slice fruit cake with a serrated knife. Serve on its own or spread with a small amount of salted butter.