Preheat oven to 350°. Spray a giant 6 muffin pan with cooking spray. Set aside. In a large bowl, combine dry ingredients. Stir in butter. Make a well in the center and pour in 1/4 cup buttermilk, eggs, zucchini and vanilla. Stir just til combined. Fold in walnuts. Note: if mixture appears too thick, add additional buttermilk, 1 T at a time. Fill muffin cups 3/4 full. Sprinkle tops with brown sugar, about 1 T on each. Bake 30-35 minutes, or til a toothpick inserted in the center comes out clean. Cool in pan before turning out onto a rack.