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

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

In a separate bowl, whisk together the egg, milk, and melted butter until smooth and slightly frothy.

Gently pour the wet ingredients into the bowl of dry ingredients and stir until just combined.

Fold in the diced firm apples gently until evenly distributed throughout the batter.

In a small bowl, combine the cold, cubed butter with brown sugar, flour, and oats; mix until it resembles coarse crumbs.

Spoon the prepared batter into the muffin liners, filling each about two-thirds full, then sprinkle the streusel topping over each muffin.

Bake in the preheated oven for 18-20 minutes until golden brown and a toothpick inserted comes out clean.

Allow the muffins to cool in the tin for about 5 minutes before transferring to a wire rack.