Preheat the oven to 350°F (175°C). Grease a 9 x 5-inch (23 x 13 cm) loaf pan and line it with parchment paper, leaving an overhang along the long sides for easier removal.

In a medium bowl, whisk together 2 cups (240 g) flour, baking powder, baking soda, 1½ teaspoons cinnamon, and salt until evenly combined. Break up any visible lumps of baking soda.

In a large bowl, whisk the melted butter and ¾ cup (150 g) brown sugar until smooth. Whisk in the eggs one at a time, followed by the sour cream and vanilla. The mixture should look smooth and creamy, though a few tiny streaks of sour cream are harmless.

Add the dry mixture to the wet ingredients: Fold gently with a spatula just until only a few streaks of flour remain. The batter will be fairly thick; avoid vigorous stirring, which can make the finished bread tough.

Fold in the diced apple and ½ cup (57 g) of the chopped pecans, reserving the remaining ¼ cup (28 g) for the streusel. Fold only until the fruit and nuts are evenly distributed and no dry flour remains.

In a small bowl, combine ¼ cup (30 g) flour, ¼ cup (50 g) brown sugar, ½ teaspoon cinnamon, and the reserved ¼ cup (28 g) pecans. Stir in 2 tablespoons (28 g) melted butter until the mixture forms moist crumbs. If it looks powdery, continue mixing until the butter is evenly distributed.

Spread the batter evenly in the prepared pan, making sure it reaches the corners. Scatter the cinnamon-pecan streusel evenly over the surface without pressing it deeply into the batter.

Bake at 350°F (175°C) for about 60 minutes. Begin checking around 55 minutes. The loaf should be well risen and golden brown, and a wooden skewer inserted into the center should emerge without wet batter; a few moist crumbs are fine. If the streusel is browning too quickly, loosely tent the loaf with aluminum foil for the final 10 to 15 minutes.

Place the pan on a wire rack and cool for 15 minutes. The loaf will still be delicate while hot, so avoid lifting it out immediately.

Use the parchment overhang to lift the loaf from the pan and place it directly on the rack. Cool for 1 hour, or until completely cooled, before slicing. The crumb will become more stable as the loaf cools.