Step 1Sift together flour, sugar, cocoa powder, baking soda and salt. Set aside.
Step 2Combine sour cream, oil, eggs and vanilla in bowl of electric mixer on low speed. Increase speed to medium and mix until light, about 2 minutes. Add flour mixture. Use low speed to mix just until well combined. Stir in zucchini. Transfer to greased and floured 13x9-inch baking pan. Smooth top.
Step 3Bake at 350 degrees until toothpick inserted in center comes out clean, about 35 minutes. Set aside to cool completely.
Step 1Place water and butter in small saucepan. Bring to boil; keep hot. Place chocolate and powdered sugar in food processor fitted with metal blade. Process until chocolate is fine (like sugar). With machine running, pour hot water mixture through feed tube. Continue processing until chocolate is melted and glaze is smooth, scraping once. Add vanilla and dash salt; pulse 1 to 2 times. Set aside until slightly thickened. Pour over cool cake. Use small knife to smooth and evenly distribute.