Preheat your oven to 350°F (175°C) and position a rack in the center. Grease a 9x5-inch loaf pan, then line with parchment paper, allowing some overhang for easy removal.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg for at least 30 seconds to ensure even distribution of spices.
Peel, core, and dice the apples into ¼-inch pieces. For textural contrast, cut one cup finer (about ⅛-inch) and leave the other cup slightly larger. Toss the diced apples with a teaspoon of the flour mixture to prevent them from sinking during baking.
In a large bowl, beat the softened butter and sugar until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract and bourbon.
Gradually fold the dry ingredients into the wet mixture, stirring just until combined. Aim for no more than 12-15 strokes with your spatula to avoid over-mixing.
Gently fold in the prepared apples and nuts (if using), ensuring the apple pieces are evenly distributed throughout the batter.
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 with a few moist crumbs. If the top is browning too quickly, tent with aluminum foil after 40 minutes.
Allow the bread to cool in the pan for 15 minutes before lifting it out using the parchment paper sling. Transfer to a wire rack and cool for at least 30 minutes more before glazing.
While the bread cools, whisk together the powdered sugar, bourbon, heavy cream, vanilla extract, and salt until smooth. Adjust the consistency if needed.
Once the bread has cooled, drizzle the bourbon glaze over the top, allowing it to cascade down the sides. For best results, apply the glaze while the bread is still slightly warm (around 100°F).