Preheat your oven to 375°F (190°C) and prepare your muffin tin.

Whisk together the all-purpose flour, granulated sugar, baking powder, ground cinnamon, and salt in a large mixing bowl.

In a separate bowl, mix together the milk, vegetable oil, and egg until fully combined.

Gently fold the wet mixture into the dry ingredients until just combined.

Fold in the diced apples, distributing them evenly throughout the batter.

Combine brown sugar, extra flour, and a pinch of cinnamon in a small bowl for the streusel topping. Cut in the cold butter until the mixture resembles coarse crumbs.

Spoon the batter into the prepared muffin tin, filling each cup about 3/4 full. Top each muffin with the streusel mixture.

Bake for 20-25 minutes until tops spring back when touched and a toothpick inserted comes out clean.

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