01 - Preheat the 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 blended.
03 - In a large bowl, whisk the melted butter, granulated sugar, and brown sugar until combined. Beat in the eggs, then stir in the 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 - In a small bowl, mix the sugar and cinnamon for the swirl until combined.
07 - Pour half the batter into the prepared loaf pan. Sprinkle half of the cinnamon-sugar mixture over the batter. Add the remaining batter and top with the rest of the cinnamon-sugar. Swirl with a knife for 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.