Preheat your oven to 350°F (175°C) and position the rack in the middle. Grease a 9x5-inch loaf pan and line with parchment paper, leaving an overhang on the sides for easy removal.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, nutmeg, and cloves until evenly distributed. Mix thoroughly for at least 30 seconds to ensure even distribution.
In a large bowl, beat the softened butter and sugars together until light and fluffy, about 3-4 minutes. The mixture should lighten in color and increase in volume by approximately 30%.
Beat in the eggs one at a time, mixing well after each addition. Then stir in the spiced rum and vanilla extract.
Gradually fold the dry ingredients into the wet mixture, being careful not to overmix. Stop when you just see the flour disappear.
Gently fold in the diced apples and nuts (if using). Consider using two different varieties of apples for more complex flavor.
Pour the batter into the prepared loaf pan and smooth the top with a spatula. Bake for 55-60 minutes, or until a toothpick inserted into the center comes out clean with a few moist crumbs. If the top starts browning too quickly, tent with aluminum foil after about 35 minutes.
While the bread is cooling, whisk together powdered sugar, spiced rum, and cinnamon until smooth. Start with 2 tablespoons of rum and add more as needed to achieve your desired consistency.
When the bread has cooled in the pan for 15 minutes, remove it using the parchment paper overhang and place on a wire rack with a baking sheet underneath. While the bread is still warm (but not hot), pour the glaze over the top, allowing it to drip down the sides.
Allow the bread to cool completely on the wire rack for an additional 30 minutes before slicing and serving.