Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or grease it well.
In a large bowl, whisk together the flour, sugar, baking powder, cinnamon, and salt.
In another bowl, whisk together the melted butter, eggs, milk, and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix until just combined. Do not overmix.
Fold in the chopped apples.
Divide the batter evenly among the muffin cups, filling each about ⅔ full.
Bake for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.