Preheat the oven to 350°F. Prepare a 9-by-5-inch loaf pan by spraying it with nonstick cooking spray or lining it with foil followed by a spray for effortless release.

Combine the brown sugar with 1 teaspoon of cinnamon in a small bowl and set it aside. In a separate bowl, toss the diced apples with 2 tablespoons of granulated sugar and the remaining 1 teaspoon of cinnamon.

In a medium bowl, beat the softened butter together with the remaining 2/3 cup of granulated sugar using an electric mixer until the mixture is smooth and creamy, about 5 minutes. Add the eggs one at a time, ensuring each is fully incorporated before adding the next. Mix in the vanilla extract.

In another bowl, whisk together the all-purpose flour and baking powder. Gradually add this dry mixture to the creamed butter mixture, beating until just combined. Stir in 1/2 cup of milk until the batter reaches a smooth consistency.

Pour half of the prepared batter into the loaf pan. Distribute half of the apple mixture evenly over the batter. Sprinkle with half of the brown sugar and cinnamon mixture.

Add the remaining batter over the apple layer, then top with the rest of the chopped apples and the remaining brown sugar-cinnamon mixture. Gently press the apples into the batter and swirl the top using a knife or spoon to create a marbled effect.

Bake for about 1 hour and 10 minutes, or until a toothpick inserted into the center of the loaf emerges clean.

To prepare the glaze, mix the powdered sugar with 1 to 3 tablespoons of milk, adjusting to achieve the desired consistency.

Once baked, allow the loaf to cool for 15 minutes before drizzling with the glaze.