Spoon one-fourth of batter into pan and spread evenly. Sprinkle one-third of grated chocolate (about 1-1/2 tablespoons) over batter. Repeat this procedure two more times. Top with remaining batter.
Bake, cool and remove cake from pan as directed on [cake mix] package.
Melt together in small saucepan, stirring constantly over low heat, 6 squares chocolate (6 ounces) and 4 tablespoons butter. Cool slightly, then drizzle over cake; sprinkle coconut on top. Store loosely covered.