Preheat oven to 350°. Grease a 13x9-in. baking pan., In a large bowl, whisk the first seven ingredients. In another bowl, whisk eggs, yogurt, oil and vanilla until blended. Add to flour mixture; stir just until moistened. Stir in zucchini., Transfer batter to prepared pan. Bake 25-30 minutes or until top springs back when lightly touched. Cool completely in pan on a wire rack., For frosting, place evaporated milk, sugar and vanilla in a blender jar; cover and process 30 seconds. In a microwave, melt chocolate and butter; stir until smooth. Add to milk mixture; cover and process 30-45 seconds or until thickened and smooth, stopping to scrape blender jar as needed.