Preheat the oven to 375°F or 190°C and line a 12-cup muffin pan with paper liners. Lightly grease the tops of the pan in case the muffins spread.
Whisk flour, baking powder, cinnamon, and salt in a medium bowl. Breaking up any small lumps now makes the batter easier to mix evenly.
In a larger bowl, whisk melted butter with sugar, eggs, vanilla, and milk. Make sure the butter has cooled slightly so it does not cook the eggs.
Add the dry ingredients to the wet mixture and fold gently until almost combined. Add the diced apples and continue folding only until no dry pockets remain.
Divide the batter among the muffin cups, filling each about three-quarters full. Mix the topping ingredients and sprinkle a little over every muffin.
Bake for 18 to 22 minutes, until the tops spring back and a toothpick comes out clean. Cool for 5 minutes in the pan before moving to a rack.
