Preheat the oven to 350. Spray a mini muffin tin with cooking spray, then set aside.

In a large bowl, whisk together the flour, sugar, baking powder, salt, and cinnamon. In a small bowl or measuring cup, whisk together the egg, milk, oil, and vanilla. Pour the wet ingredients into the dry ingredients, then mix until just combined. Gently fold in the apple.

Scoop the batter into the prepared muffin tin, filling nearly to the top. Bake 14-16 minutes, or until golden brown on the edges. Remove from the oven, then let cool slightly.

Whisk together the sugar and cinnamon. Dip the top of each muffin in the melted butter, then in the cinnamon sugar. Allow to cool, and then enjoy!