Preheat the oven to 325°F (165°C). Line a muffin pan with liners and lightly spray with non-stick cooking spray; set aside.
In a large bowl, whisk together the honey, egg, vanilla and Greek yogurt until smooth.
Stir the cooled melted butter into the wet mixture.
Grate the zucchini and blot or squeeze to remove excess moisture, then stir the zucchini into the wet ingredients.
In a separate bowl, whisk together the whole wheat pastry flour, vanilla protein powder, nutmeg, cinnamon, kosher salt and baking soda.
Fold the dry ingredients into the wet ingredients until just combined, then gently fold in the chopped walnuts if using. Do not overmix.
Spoon the batter into the prepared muffin liners, filling each about 3/4 full.
Bake for 15–17 minutes, or until a toothpick inserted into the center comes out clean. Cool before serving.