TO MAKE THE CAKE: Preheat the oven to 350^F. Generously coat a Bundt pan with nonstick spray.
In a medium bowl, whisk together the flour, sugar, cocoa, baking powder, orange peel, baking soda, cinnamon, and salt. Set aside.
In a large bowl, using an electric mixer set on high speed, beat the eggsand egg whites for 2 minutes, or until frothy and thickened. Add the buttermilk, applesauce, and vanilla extract. Beat just until blended.
Gradually beat in half of the flour mixture. Beat in the remaining flour mixture. Stir in the zucchini and walnuts. Scrape into the preapared pan.
Bake for 45 to 50 minutes, or until a wooden pick inserted in the center comes out clean.
Cool in the pann on a rack for 15 minutes. Loosen the edges of the cake by gently inserting a thin rubber spatula or knife around the sides and center. Invert the cake onto the rack to cool completely.
TO MAKE THE GLAZE: In a small microwaveable cup, combine the chocolate chips and oil. Cook in the microwave on high for 30 seconds and stir. Repeat just until the chocolate can be stirred into a smooth glaze. (The chips will not melt completely in the microwave, but they will get shiny and soft enough to stir until smooth.) Drizzle the warm glaze over the cooled cake.