Step 1Heat the oven to 325 degrees. In a large bowl, combine the egg whites, vanilla extract, orange zest and vegetable oil, and whisk just to combine.
Step 2Whisk in 3/4 cup sugar and the salt just to combine.
Step 3In a medium bowl, whisk together the cinnamon, baking soda, baking powder and flour. Gently whisk the dry ingredients into the egg white mixture just until combined. Fold in the zucchini and walnuts.
Step 4Grease an 8 1/2-inch by 4 1/2-inch by 2 1/2-inch metal loaf pan. Line the pan with parchment paper and grease the paper. Pour the batter into the pan.
Step 5Lightly sprinkle the top of the batter with the remaining 1 1/2 teaspoons sugar.
Step 6Bake until the cake is set (it will spring back when gently touched on top, and a knife inserted will come out clean), about 1 to 1 1/2 hours; ours typically set after 1 hour and 15 minutes, though this can vary by pan and heat of the oven.
Step 7Cool the loaf for a few minutes on a rack, then remove from the baking dish and cool completely on the rack.