Preheat your oven to 350°F (175°C).
Line a muffin tin with paper liners.
In a large mixing bowl, combine the dry ingredients: flour, sugar, baking powder, baking soda, salt, and cinnamon.
In a separate bowl, whisk together the egg, melted butter, and milk.
Pour the wet mixture into the dry ingredients and stir until just combined.
Fold in the diced apples and nuts if using.
Scoop the batter into the prepared muffin tin, filling each cup about ¾ full.
Bake for 18-20 minutes or until a toothpick inserted comes out clean.
Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire rack.
Serve warm or at room temperature.
