Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, cinnamon, and salt until well combined.

In a separate bowl, mix the melted butter, egg, and milk until smooth.

Pour the wet ingredients into the dry mixture and stir gently until just combined; be careful not to overmix.

Finally, fold in the peeled and diced apples, ensuring they are evenly distributed.

In a small bowl, combine cold butter, brown sugar, flour, cinnamon, and optional oats.

Use a fork or your fingers to mix these ingredients together until the mixture resembles coarse crumbs.

Set aside while you prepare to fill the muffin tins.

Spoon the muffin batter evenly into the prepared muffin tins, filling each liner about three-quarters full.

Generously sprinkle the streusel topping over each muffin, ensuring a good distribution for optimal texture and flavor.

Press the streusel lightly into the batter to help it adhere during baking.

Place the muffin tin in the preheated oven and bake for 18-20 minutes.

You’ll know the muffins are done when they turn a lovely golden brown and a toothpick inserted into the center comes out clean.

Allow muffins to cool in the tin for about 5-7 minutes before transferring them to a wire rack to cool completely.