1. Preheat your oven to 375°F (190°C). Line a 12-cup muffin pan with paper liners.

2. In a large bowl, whisk together the flour, granulated sugar, baking powder, cinnamon, and salt.

3. In a separate medium bowl, whisk the egg, milk, melted butter, and vanilla extract until combined.

4. Pour the wet ingredients into the dry ingredients. Stir just until moistened. Do not overmix; a few lumps are fine.

5. Gently fold in the chopped apples.

6. For the streusel topping: In a small bowl, combine the flour, brown sugar, and cinnamon. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs.

7. Divide the muffin batter evenly among the 12 muffin cups.

8. Sprinkle a generous amount of streusel topping over each muffin.

9. Bake for 18-22 minutes, or until a wooden skewer inserted into the center of a muffin comes out clean.

10. Let the muffins cool in the pan for a few minutes before transferring them to a wire rack to cool completely.