Preheat the oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large mixing bowl, combine the flour, sugar, baking powder, cinnamon, and salt.
In another bowl, whisk together the melted butter, egg, and milk until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the diced apples and nuts if using.
Scoop the batter into the prepared muffin tin, filling each liner about two-thirds full.
Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
Once baked, allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack.