Preheat oven to 400°F. In a large bowl, combine the flour, baking powder, baking soda, and salt, then set aside.
In a separate bowl, whisk together the melted butter and sugars.
Whisk the milk, eggs, and vanilla extract into the butter mixture until well combined.
Fold in the dry ingredients, mixing until just combined. It's okay if there are still a few lumps.
Fold in the apples and cinnamon. Avoid overmixing.
Line a 12-count muffin tin with paper liners or grease heavily. Fill each muffin cup ¾ full.
Bake in the preheated oven for 5 minutes, then turn the temperature down to 350°F. Bake for 16-17 more minutes until the centers are completely set, verified with a toothpick inserted in the center.
Allow the muffins to cool for five minutes in the pan, then move to a wire rack to finish cooling.
Notes
Make sure your butter is fully melted before adding it to the batter. This step, plus the high temperature at the start, gives the muffins that perfect domed top!
Only allow the muffins to cool in the pan for 5 minutes, then move them to a wire rack to finish cooling—that's how you dodge dry muffins.
Make sure not to overmix the batter, or you will end up with dry, dense muffins. Stir the wet and dry ingredients together until they are just combined. It's okay if there are a few lumps left!