Heat the oven to 350°F (175°C). Grease the loaf pan and add a parchment sling. Peel, core, and finely dice the apples.
Whisk flour, baking powder, baking soda, salt, and batter cinnamon. Mix the separate topping sugar and cinnamon in a small bowl.
Whisk brown sugar, eggs, oil, yogurt, and vanilla in another bowl until smooth. Keep the diced fruit aside until the batter is nearly combined.
Fold dry ingredients into wet just until a few streaks remain. Fold in apples gently, reaching the bottom of the bowl without beating the batter.
Spread evenly in the pan and scatter cinnamon sugar over the top. Do not swirl the topping into the batter, since it is intended as a surface finish.
Bake 50–60 minutes until a center tester shows no raw batter and apple pieces are tender. Shield the top if it darkens too early.
Cool in the pan 15 minutes, then lift onto a rack and cool about 1 hour. Avoid wrapping while warm, which traps moisture against the crust.
Slice with a serrated knife and support the crumb with a board. Keep the remaining cut end covered and store only after the loaf is completely cool.
