Preheat your oven to 350°F (175°C). Prepare a 12-cup muffin tin by lining it with paper liners or lightly coating with butter.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon until everything is well blended.
In a separate bowl, stir the granulated sugar, brown sugar, melted butter, eggs, vanilla, and milk until smooth and combined.
Gently fold the dry mixture into the wet mixture, mixing only until no streaks of flour remain. Avoid overmixing so the muffins stay light.
Stir in the diced apples carefully with a spatula so they’re evenly distributed throughout the batter.
For the crumb topping, mix the flour, brown sugar, and cinnamon in a small bowl. Cut in the softened butter using your fingers or a fork until the mixture looks like coarse crumbs.
Spoon the batter into the muffin cups, filling them about ¾ full. Top each one generously with the crumb mixture.
Bake for 18–22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Let the muffins cool in the pan for 10 minutes, then move them to a wire rack to finish cooling completely.