Preheat the oven to 350°F (175°C) and line a muffin tin with paper liners.

In a mixing bowl, whisk together the dry ingredients: flour, sugar, baking powder, baking soda, cinnamon, and salt.

In another bowl, combine the wet ingredients: melted butter, buttermilk, eggs, and vanilla extract.

Pour the wet ingredients into the dry ingredients and mix until just combined.

Fold in the diced apples and walnuts, if using.

Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full.

Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.

Allow the muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.