1. Prepare the streusel topping by mixing flour, brown sugar, and cinnamon, then stirring in melted butter until crumbly. Set aside.

2. In a bowl, whisk together flour, baking powder, salt, and cinnamon.

3. In another bowl, cream together the softened butter and sugar until light and fluffy.

4. Beat in the eggs one at a time, then mix in the vanilla extract.

5. Stir in the sour cream until smooth and combined.

6. Gradually add the dry mixture to the wet mixture, mixing until just combined.

7. Fold in the chopped apples evenly.

8. Pour half the batter into a greased loaf pan, optionally sprinkle cinnamon sugar, then add the rest of the batter. Swirl gently with a knife.

9. Sprinkle the streusel topping evenly over the batter and press down slightly.

10. Bake at 350°F (175°C) for 50–60 minutes. Cover loosely with foil if browning too fast.

11. Let cool 10–15 minutes in the pan, then transfer to a wire rack. Once cooled, drizzle with glaze if desired.