Preheat your oven to 350°F. Grease a 9x5-inch loaf pan and line it with parchment paper for easier removal later.

In a small bowl, toss the diced apples with 1/3 cup granulated sugar and 1 teaspoon cinnamon. Set this aside to let the apples get nice and juicy while you prepare the batter.

In a large mixing bowl, cream the softened butter and 2/3 cup sugar together until light and fluffy. Beat in the eggs one at a time, followed by the vanilla.

In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add these dry ingredients to the wet ingredients, alternating with the milk. Mix just until combined, being careful not to overmix.

Pour half of the batter into your prepared loaf pan, spreading it out evenly. Spoon half of the apple mixture over the batter, followed by half of the brown sugar and cinnamon mixture.

Use a knife to gently swirl the layers together. Repeat with the remaining batter, apples, and cinnamon-sugar mixture, swirling the top layer again for a pretty finish.

Bake the bread in the preheated oven for 50–55 minutes, or until a toothpick inserted into the center comes out clean. If the top starts to brown too quickly, loosely cover the loaf with aluminum foil during the last 10 minutes of baking.

Let the bread cool in the pan for about 15 minutes, then transfer it to a wire rack to cool completely. While the bread cools, whisk together the powdered sugar, milk, and vanilla to make the glaze. Drizzle the glaze over the cooled loaf before slicing and serving.