01 - Preheat oven to 350°F. Grease and flour a 9x5-inch loaf pan or line with parchment paper.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon until well combined.
03 - In a large bowl, whisk melted butter, granulated sugar, and brown sugar until smooth. Beat in eggs, then stir in milk and vanilla extract.
04 - Gradually fold dry ingredients into wet mixture until just combined. Do not overmix.
05 - Gently fold diced apples into batter.
06 - In a small bowl, mix sugar and cinnamon for swirl topping.
07 - Pour half the batter into prepared pan. Sprinkle with half cinnamon-sugar mixture. Add remaining batter and top with remaining cinnamon-sugar. Swirl with knife for marbled effect.
08 - Bake for 50-55 minutes until toothpick inserted in center comes out clean.
09 - Let bread cool in pan for 10 minutes, then transfer to wire rack to cool completely before slicing.