Preheat oven to 400°. Line muffin tins and set aside.
Melt butter and set aside to cool. Meanwhile, cut zucchinis in half. Scrape any seeds out of the middle, and grate using the large holes of a box grater. Press a paper towel into zucchini to absorb excess moisture.
In a large bowl, whisk together flours, baking soda, baking powder, cinnamon, salt, and nutmeg.
Into cooled butter, whisk in sugar, eggs, yogurt, orange juice, and extract.
Add butter mixture, zucchini, and walnuts to flour mixture. Stir until just combined.
Mix together crumble with fork or hand until combined.
Evenly divide batter into muffin tins. Top with crumble. Bake for 20 minutes. Allow to cool before serving.