Preheat & Prep Pan: Preheat your oven to 350°F (175°C). Line an 8.5x4.5-inch or 9x5-inch metal loaf pan with parchment paper, leaving an overhang on the sides for easy removal, and lightly spray with nonstick cooking spray.

Toss Apples: In a medium bowl, combine the diced apples, ⅓ cup dark brown sugar, cinnamon, and nutmeg. Stir until the apples are glistening and evenly coated in the spiced sugar. Set aside.

Cream Butter & Sugar: In a large mixing bowl (or stand mixer fitted with the paddle attachment), beat the softened butter and granulated sugar on medium-high speed for 2 to 3 minutes until pale, light, and fluffy.

Add Eggs & Vanilla: Beat in the eggs one at a time, incorporating fully before adding the next. Stir in the vanilla extract until smooth.

Alternate Dry & Wet: Whisk together flour, baking powder, and salt. Add half of the flour mixture to the creamed butter, mixing on low speed just until combined. Pour in the milk, followed by the remaining flour mixture, mixing gently until no flour streaks remain. Do not overmix.

Layer & Swirl: Spread half of the thick loaf batter into the prepared pan. Scatter half of the cinnamon-spiced apples evenly over the surface. Spoon the remaining batter on top, followed by the rest of the apples. Using a small butter knife, gently swirl in a figure-eight motion 3 or 4 times to distribute the spiced ribbons throughout the loaf.

Bake to Golden Perfection: Bake at 350°F (175°C) for 50 to 55 minutes, or until a wooden skewer inserted into the center comes out clean or with a few moist crumbs (test the cake crumb, not a soft apple piece). If the top browns too quickly after 35 minutes, tent loosely with a sheet of aluminum foil.

Cool Completely: Let the loaf rest in the pan for 15 minutes, then use the parchment sling to transfer it to a wire cooling rack to cool completely for at least 45 minutes before glazing.

Drizzle Glaze: Whisk powdered sugar, milk, and vanilla until smooth and glossy. Drizzle generously over the cooled loaf, allowing it to cascade down the crust. Let set for 10 minutes, slice with a serrated knife, and enjoy!