Preheat your oven to 350°F (175°C) and prepare your muffin tin with liners or cooking spray.

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

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

Gently fold the wet ingredients into the dry ingredients until just combined, being careful not to overmix.

Fold in the diced apples until evenly distributed.

For the streusel topping, mix together the cold butter, brown sugar, oats, and a bit of flour until it resembles coarse crumbs.

Spoon the muffin batter into the prepared muffin tin, filling each cup about two-thirds full and topping with streusel.

Bake for 18-20 minutes, checking for doneness with a toothpick.

Let the muffins cool in the pan for about 5 minutes before transferring them to a wire rack.