Preheat oven to 350°F (180°C). Spray a 9x5-inch loaf pan with nonstick cooking spray or line with parchment paper.

In a large mixing bowl, whisk together the flour, baking powder, and salt.

In a medium-sized bowl, stir together ⅓ cup granulated sugar and 1 ½ teaspoons ground cinnamon. Set aside.

Peel and dice the apple into ½-inch cubes.

In the bowl of a stand mixer (or with an electric hand mixer), cream the butter until light and fluffy. Add granulated sugar, then brown sugar, mixing well after each addition. Scrape the sides of the bowl as needed.

Beat in eggs one at a time until the mixture is light and glossy.

Combine whole milk and vanilla. Add to the butter mixture, alternating with the flour mixture, beginning and ending with the dry ingredients. Mix just until combined.

Spread half of the batter into the prepared loaf pan. Sprinkle with half the cinnamon mixture and half the diced apples. Repeat with remaining batter, apples, and cinnamon mixture.

Bake in preheated oven for 50–60 minutes, or until a toothpick inserted in the center of the loaf comes out clean.

Let the bread cool in the pan for 10 minutes. Remove from pan and transfer to a wire rack to cool completely.

In a small bowl, whisk together powdered sugar and milk until smooth. Drizzle glaze over the cooled bread.