Preheat the oven to 350°. Line a muffin pan with paper liners or spray with nonstick cooking spray.
Peel, core, and dice the apples.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon.
In another large mixing bowl, beat together the sugar, brown sugar, melted butter, eggs, vanilla extract, and milk until well combined.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Avoid overmixing.
Gently fold in the diced apples.
In a small bowl, combine the flour, brown sugar, and cinnamon for the crumb topping. Add the softened butter and mix with a fork or your fingers until the mixture looks like coarse crumbs.
Divide the muffin batter evenly among the muffin cups, filling each about three-quarters full.
Sprinkle the crumb topping over each muffin.
Bake for 18-22 minutes or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the pan for a few minutes and then transfer them to a wire rack to cool completely.
Notes
Storage: Store leftover muffins in an airtight container at room temperature for up to three days. For longer storage, keep them in the fridge for up to a week. You can also freeze muffins in a freezer bag for up to two months.