Preheat your oven to 350°F (175°C) and prepare a muffin tin by lining it with paper liners.

In a medium bowl, mix together rolled oats, brown sugar, and cold butter until the mixture resembles coarse crumbs.

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

In another bowl, beat together the melted butter, egg, and milk until smooth.

Pour the wet mixture into the dry ingredients and gently fold them together until just combined.

Carefully fold in the chopped apples.

Scoop the batter into the prepared muffin cups, filling them about two-thirds full.

Sprinkle the prepared streusel topping over each muffin.

Bake for about 20 minutes or until the tops are golden and a toothpick inserted into the center comes out clean.

Once baked, remove the muffins from the oven and let them cool in the tin for about 5 minutes.