01 - Preheat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or lightly grease with oil spray.
02 - In a large bowl, whisk together the Greek yogurt, eggs, honey or maple syrup, oil, and vanilla extract until smooth.
03 - In a medium bowl, blend together the whole wheat flour, baking powder, baking soda, cinnamon, and salt.
04 - Gently stir the dry ingredients into the wet mixture just until combined; avoid overmixing.
05 - Fold in the diced apples and nuts, if using, until evenly distributed throughout the batter.
06 - Divide the batter evenly into the prepared muffin cups, filling each approximately three-quarters full.
07 - Bake for 18 to 22 minutes, or until a toothpick inserted in the center emerges clean.
08 - Allow muffins to cool in the tin for 5 minutes, then transfer onto a wire rack to cool completely.