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 one at a time, then stir in milk and vanilla extract.
04 - Gradually fold dry ingredients into the wet mixture until just combined. Do not overmix.
05 - Gently fold diced apples into the batter until evenly distributed.
06 - In a small bowl, mix sugar and cinnamon 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. Use a knife to gently swirl for a marbled effect.
08 - Bake for 50-55 minutes, or until a toothpick inserted in the center comes out clean.
09 - Let bread cool in pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.