Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, cinnamon, and salt.
In another bowl, combine the melted butter, eggs, and buttermilk.
Pour the wet ingredients into the dry ingredients and mix until just combined.
Fold in the diced apples gently to avoid overmixing.
Distribute the batter evenly among the muffin cups, filling each about 3/4 full.
Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Remove from the oven and let cool in the pan for 5 minutes.
Transfer muffins to a wire rack to cool completely.
Enjoy warm or store for later!