Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or spray with non-stick cooking spray.

In a large mixing bowl, sift together the flour, sugars, baking powder, cinnamon, and salt. Whisk until well combined.

In another bowl, combine melted butter, egg, and milk, whisking until blended. Pour into the dry ingredients and mix gently until no dry flour remains.

Fold in the diced apples into the batter until evenly distributed.

For the streusel, combine cold butter, flour, and brown sugar in a bowl, mixing until you have coarse crumbs.

Spoon the muffin batter into the prepared muffin tin, filling each liner about two-thirds full. Sprinkle the streusel topping over the batter.

Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. The muffins should be golden brown.

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