Preheat the oven to 375 °F. Place muffin liners in the wells of a muffin pan (if desired) or spray the muffin pan with nonstick cooking spray. In a medium bowl, whisk together the flour, sugar, and cinnamon. Add the butter, milk, beaten egg, vanilla, and chopped apple. Stir until the ingredients are moistened, but don't over mix. Spoon the mixture into the muffin pan.