Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, cinnamon, and salt.
In another bowl, mix buttermilk, vegetable oil, and eggs until well combined.
Pour the wet ingredients into the dry ingredients and stir gently until just combined.
Fold in the diced apples and walnuts if using.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Allow muffins to cool in the pan for 5 minutes before transferring to a wire rack.
Optional: drizzle with icing or sprinkle with additional cinnamon sugar.
Enjoy your warm apple cinnamon muffins with a cup of coffee or tea!