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