Preheat oven to 350°F with rack in the middle. Grease a 9x5 loaf pan and line with parchment paper, leaving overhang on long sides.
Cook the apples: Peel and dice apples into ½-inch cubes (about 1 cup). In a small skillet over medium heat, melt 1 tablespoon butter. Add apples, 1 tablespoon brown sugar, and ½ teaspoon cinnamon. Cook 3–4 minutes until soft and juices are thick. Transfer to plate and let cool completely.
Mix dry ingredients: In a large bowl, whisk together flour, baking powder, baking soda, salt, and 2 teaspoons cinnamon.
Mix wet ingredients: In a medium bowl, whisk together oil, granulated sugar, brown sugar, eggs, buttermilk, and vanilla until smooth and thickened, about 30 seconds.
Combine: Pour wet into dry; stir with rubber spatula just until no streaks remain. Lumps are fine – do not overmix.
Layer and swirl: Spread half the batter in pan. Top with half the cooled apples and half the swirl sugar (1 tbsp cinnamon + 2 tbsp brown sugar mixed). Spoon remaining batter, then remaining apples and swirl sugar. Gently make two figure-eight motions with a knife.
Add crumb topping: Sprinkle evenly over batter, pressing lightly. For best results, chill topping 10 minutes before adding so it stays clumpy.
Bake 55–65 minutes until toothpick comes out clean or with a few moist crumbs. Tent with foil at 40 minutes if top browns too quickly. Let cool in pan 15 minutes, then lift onto wire rack. Cool completely before slicing – at least 1 hour.