Preheat the oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
In a medium bowl, whisk together the whole wheat flour, baking powder, baking soda, ground cinnamon, ground nutmeg, and salt.
In a large bowl, beat the eggs until frothy. Add the unsweetened applesauce, pure maple syrup, plain Greek yogurt, and vanilla extract. Mix until well combined.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined. Do not overmix.
Fold in the diced apples and chopped walnuts (if using) gently into the batter.
Pour the batter into the prepared cake pan and spread evenly.
Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool in the pan for 10 minutes.
Transfer the cake to a wire rack to cool completely before slicing and serving.