Preheat oven to 350°F (175°C). Grease and flour a 9×5-inch loaf pan or line it with parchment.
Prepare cinnamon swirl: In a small bowl, mix the brown sugar and cinnamon. Set aside.
Make the batter: In a large bowl, cream together the white sugar and softened butter until fluffy. Beat in eggs one at a time, then add vanilla.
Add dry ingredients: In a separate bowl, mix flour and baking powder. Gradually add to the wet mixture, alternating with the milk. Mix until just combined.
Fold in apples: Gently stir in the chopped apples.
Layer and swirl: Pour half the batter into the loaf pan. Sprinkle with half the cinnamon sugar mix. Add remaining batter on top. Finish with the rest of the cinnamon sugar. Use a knife to gently swirl through the batter (a figure-8 motion works well).
Bake: Bake for 50–60 minutes, or until a toothpick inserted comes out clean. Let cool in the pan for 10–15 minutes, then transfer to a wire rack.
