Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan.

In a large bowl, whisk together the flour, baking powder, salt, and ground cinnamon until well combined.

In a separate bowl, mix the granulated sugar, vegetable oil, eggs, vanilla extract, and milk until smooth.

Gradually add the wet ingredients to the dry ingredients, stirring gently until combined. Do not overmix.

Fold in the diced apples, ensuring they are evenly distributed in the batter.

In a small bowl, combine the brown sugar and the additional tablespoon of ground cinnamon.

Pour half of the batter into the prepared loaf pan and sprinkle half of the cinnamon-sugar mixture on top.

Pour the remaining batter on top, followed by the remaining cinnamon-sugar mixture. Use a knife to swirl the topping gently into the batter to create a marbled effect.

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

Once baked, allow the bread to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.

For a delicious finishing touch, whisk together powdered sugar and milk to create a glaze and drizzle it over the cooled bread, if desired.