Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.

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

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

Fold the wet mixture into the dry ingredients until just combined, then gently incorporate the diced apples.

For the streusel, mix cold butter, brown sugar, oats, and cinnamon until crumbly.

Fill the muffin cups three-quarters full with batter, then sprinkle streusel on top.

Bake for 20 minutes or until golden brown and a toothpick comes out clean.

Cool in the pan for 10 minutes before transferring to a wire rack.