Preheat the oven to 350°F (175°C) and grease the muffin tin.

In a large mixing bowl, combine the dry ingredients: flour, sugar, baking powder, cinnamon, and salt.

In another bowl, whisk together the wet ingredients: melted butter, milk, eggs, and vanilla extract.

Pour the wet ingredients into the dry ingredients and mix until just combined.

Fold in the diced apples gently.

Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full.

Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.

Allow the muffins to cool in the tin for 5 minutes before transferring to a wire rack.