Step 1: Preheat oven to 350°F (175°C) and grease a 9×5-inch loaf pan.
Step 2: In a medium bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
Step 3: In a large bowl, cream butter and sugar until light and fluffy, about 2 minutes.
Step 4: Beat in eggs one at a time, then mix in vanilla extract and sour cream.
Step 5: Gradually add dry ingredients to wet ingredients, mixing until just combined.
Step 6: Fold in diced apples, then pour batter into prepared loaf pan.
Step 7: For streusel, combine flour, brown sugar, granulated sugar, and cinnamon in a small bowl.
Step 8: Cut in cold butter with a fork or pastry cutter until mixture resembles coarse crumbs.
Step 9: Sprinkle streusel evenly over the batter in the pan.
Step 10: Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Step 11: Let cool in pan for 15 minutes, then transfer to a wire rack to cool completely.