Preheat your oven to 425°F (220°C). Line a 6-cup muffin tin with paper liners or grease the cups.

In a small bowl, combine streusel ingredients: brown sugar, flour, cinnamon, and butter. Use your fingertips to pinch until crumbly. Set aside.

In a large bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg.

In another bowl, beat the egg with granulated sugar, brown sugar, milk, oil, and vanilla until smooth.

Fold the wet ingredients into the dry ingredients just until combined. Gently stir in the diced apples—be careful not to overmix.

Divide the batter evenly among the muffin cups. Sprinkle a generous amount of streusel over each muffin.

Bake for 5 minutes at 425°F, then reduce oven temperature to 350°F (175°C) and bake an additional 20–22 minutes, or until a toothpick comes out clean.

Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.