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 combined.
03 - Whisk melted butter, granulated sugar, and brown sugar in a large bowl until smooth. Beat in eggs, then stir in milk and vanilla extract.
04 - Gradually fold the dry ingredients into the wet mixture until just combined. Do not overmix.
05 - Gently fold in the diced apples until evenly distributed throughout the batter.
06 - Mix the sugar and cinnamon for the swirl in a small bowl until combined.
07 - Pour half the batter into the 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 a marbled effect.
08 - Bake for 50 to 55 minutes, or until a toothpick inserted in the center comes out clean.
09 - Let the bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.