Preheat the oven to 375°F (190°C). Grease or line a 12-cup muffin pan.
In a medium bowl, whisk together the flour, baking powder, salt, and cinnamon.
In another bowl, toss the diced apples with the extra flour and cinnamon until evenly coated.
In a large mixing bowl, cream the softened butter and sugar for about 3 minutes until light and fluffy.
Beat in the eggs one at a time, followed by the vanilla extract.
Gradually add the dry ingredients, alternating with the milk. Mix only until combined.
Gently fold the coated apples into the batter.
Divide the batter evenly among the muffin cups, filling each about ⅔ to ¾ full.
Bake the Cinnamon Apple Muffins for 20–30 minutes, or until a toothpick inserted into the center comes out clean.
While the muffins bake, melt the butter for the topping and combine the sugar and cinnamon in a shallow bowl.
Allow the muffins to cool for 5 minutes before removing them from the pan.
Brush the tops with melted butter, then dip each muffin into the cinnamon sugar mixture until well coated.
Cool slightly before serving.
