01 - Preheat the oven to 375°F. Line a 12-cup muffin pan with paper liners or lightly coat with oil spray.
02 - In a large bowl, whisk together Greek yogurt, eggs, honey or maple syrup, oil, and vanilla extract until smooth.
03 - In a separate bowl, stir together whole wheat flour, baking powder, baking soda, ground cinnamon, and salt.
04 - Add dry mixture to wet mixture and gently stir until just combined, taking care not to overmix.
05 - Fold in diced apples and chopped nuts if using, distributing evenly throughout the batter.
06 - Spoon the batter evenly into the prepared muffin cups, filling each about three-quarters full.
07 - Bake for 18 to 22 minutes, or until a toothpick inserted in the center of a muffin comes out clean.
08 - Let muffins cool in the tin for 5 minutes, then transfer to a wire rack and allow to cool completely.