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

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

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

Pour the wet ingredients into the dry ingredients. Stir just until moistened; a few lumps are fine.

Gently fold in the chopped apples.

For the streusel topping: In a small bowl, combine the flour, brown sugar, and cinnamon. Cut in the cold butter until the mixture resembles coarse crumbs.

Divide the muffin batter evenly among the muffin cups.

Sprinkle streusel topping over each muffin.

Bake for 18-22 minutes, or until a skewer inserted comes out clean.

Let cool in the pan for a few minutes before transferring to a wire rack.