Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large bowl, mix together the flour, sugar, baking powder, cinnamon, and salt.
In a separate bowl, whisk together the melted butter, milk, and eggs.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the diced apples and nuts if using.
Fill each muffin cup about 2/3 full with batter.
Bake for 20-25 minutes or until a toothpick inserted comes out clean.
Allow the muffins to cool in the tin for 5 minutes, then transfer to a wire rack.
Enjoy warm or at room temperature!
Store any leftovers in an airtight container.