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

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

In another bowl, combine the melted butter, egg, milk, and chopped apples. Mix well.

Fold the wet ingredients into the dry ingredients gently until just combined.

In a small bowl, mix oats, flour, brown sugar, and cold butter until crumbly.

Fill muffin cups with batter and sprinkle the streusel topping generously over each.

Bake for 18-20 minutes, performing the toothpick test to check for doneness.

Cool the muffins in the pans for about 5 minutes before transferring to a wire rack.