Preheat the oven to 375°F (190°C) and prepare a muffin tin with paper liners.

Peel and grate enough Granny Smith apple to produce approximately 1 cup of shredded fruit. One large apple will generally provide the required amount, although two smaller apples may also be used.

In a medium mixing bowl, thoroughly whisk together the all-purpose flour, baking powder, granulated sugar, cinnamon, and salt. Set the dry mixture aside.

In a separate bowl, whisk the milk, room-temperature egg, and melted butter until the mixture is smooth and evenly blended.

Add the shredded Granny Smith apple to the liquid mixture and stir gently until the fruit is distributed throughout.

Pour the apple and wet-ingredient mixture into the bowl containing the dry ingredients.

Using a fork, carefully fold the mixture together by lifting the batter from the bottom and turning it over the top. Continue only until the ingredients are just combined. A few remaining traces of flour are acceptable; excessive mixing may produce firm or tough muffins.

Divide the batter evenly among the prepared muffin cups.

Bake at 375°F (190°C) for 18–25 minutes, until the muffin tops begin to turn lightly golden and a toothpick inserted into the center comes out clean.

Remove the muffins from the oven and allow them to cool before serving.