01 - Preheat the oven to 375°F. Place paper liners in a 12-cup muffin tin or lightly coat with oil spray.
02 - Whisk Greek yogurt, eggs, honey or maple syrup, oil, and vanilla extract in a large mixing bowl until smooth.
03 - In a separate bowl, stir together whole wheat flour, baking powder, baking soda, ground cinnamon, and salt.
04 - Gently fold the dry mixture into the wet ingredients, mixing just until blended. Avoid overmixing.
05 - Add diced apples and chopped walnuts or pecans if using. Fold into the batter.
06 - Spoon the batter evenly into prepared muffin cups, filling each about three-quarters full.
07 - Bake for 18 to 22 minutes, or until a toothpick inserted in the center comes out clean.
08 - Allow muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.