01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly coat with oil spray.
02 - In a large mixing bowl, whisk together Greek yogurt, eggs, honey or maple syrup, oil, and vanilla extract until smooth.
03 - In a medium bowl, stir together whole wheat flour, baking powder, baking soda, ground cinnamon, and salt.
04 - Add the dry ingredients to the yogurt mixture and stir gently until just combined. Avoid overmixing.
05 - Gently fold in the diced apples and chopped nuts, if using.
06 - Divide the batter evenly among the 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 - Let muffins rest in the tin for 5 minutes, then transfer to a wire rack to cool completely.