Preheat oven to 350°F (175°C). Grease & flour a 9x5-inch loaf pan. Mix ¼ cup sugar + 1½ tsp cinnamon for swirl & topping.
Whisk flour, baking powder, baking soda, 1 tsp cinnamon, and salt in a large bowl. In another bowl, whisk eggs, granulated sugar, brown sugar, oil, and vanilla. Fold wet into dry until just combined. Toss apples in 1 tbsp flour and fold into batter.
Pour half the batter into pan, sprinkle half cinnamon-sugar, top with remaining batter and cinnamon-sugar. Drizzle 2 tbsp melted butter.
Bake 50–60 mins until toothpick comes out with moist crumbs. Tent with foil after 40 mins if browning too fast.
Cool 15 mins in pan, transfer to wire rack. Serve warm or room temperature.
Notes
Chop apples into ¼-inch pieces for even baking and toss in 1 tbsp flour to keep suspended. Avoid overmixing for tender crumb.