Prepare your muffin pan by lining it with liners and greasing the liners generously with butter; set aside.
Mix the flour, cinnamon, salt, and baking powder in a bowl; set aside. This is the dry mixture.
Add butter and both the sugars in a large bowl and beat, using an electric hand mixer at medium-high speed, until the mixture turns light and creamy. This might take around 2-3 minutes.
Add the eggs and vanilla to the butter-sugar mixture and beat on medium speed, until well combined. This is the wet mixture.
Gently fold the dry mixture into the wet mixture alternating the milk and the dry mix. Mix just until the flour disappears. Do not over mix as that can make the muffins dense.
Toss the diced apple with ½ Tablespoon of flour and ½ teaspoon of cinnamon; doing so will keep them from sinking to the bottom while baking. Reserve 4 Tablespoons of the apples and set aside.
Gently fold the non-reserved apples into the batter using a silicone spatula until evenly distributed. Do not over mix.
Divide the batter between the muffin cups, filling almost to the top. Press the reserved apples on the top of each.
Preheat the oven to 425° F and let the batter rest while the oven preheats.
Bake at 425° F for 6 minutes, then, without opening the oven door, reduce the temperature to 375° F and bake for another 18-20 minutes, or until the muffins turn golden brown on top and a toothpick inserted into the middle comes out clean.
Allow the muffins to cool in the pan for 3-4 minutes before coating them with cinnamon-sugar.
Mix ¼ cup sugar with 1 Tablespoon of ground cinnamon (you can increase or decrease the amount of cinnamon as per your liking). pour the mixture onto a plate or shallow dish.
Remove the muffins from the pan and brush the melted butter on top and then dip them into the cinnamon-sugar mixture while they are still warm.
Serve while still a little warm, or cooled.