Preheat the oven to 350°F. Grease and flour a 9 x 13-ionch pan.
Sift together flour, baking powder, baking soda, salt, cinnamon, nutmeg and cocoa powder; set aside.
Beat the eggs with the sugar and oil until very light. Stir in the buttermilk or yogurt. Gently blend in the dry ingredients until thoroughly mixed, then stir in the zucchini, walnuts, vanilla and orange oil.
Spoon the mixture into the prepared pan, and bake for 50 to 60 minutes, or until the cake tests done. Remove the cake from oven and cool slightly on a wire rack. While the cake is cooling, make the glaze.
Mix together all the ingredients, beating till smooth. Drizzle the glaze over the cake while it's still slightly warm.