Prepare cake batter, in 2-1/2-qt. ovenproof bowl, as directed on package; scrape side of bowl.
Bake 1 hour 5 min. or until toothpick inserted in center comes out clean. Cool cake 10 min.
Loosen cake from side of bowl with knife. Invert onto wire rack; remove bowl. Cool cake completely. Meanwhile, microwave 4 oz. chocolate in large microwaveable bowl as directed on package. Cool.
Add 1 tub COOL WHIP to melted chocolate; whisk until blended.
Cut cake into 3 layers; stack on plate, spreading chocolate filling between layers.
Frost cake with remaining COOL WHIP. Make curls from remaining chocolate. (See Tip.)
Place on cake; sprinkle with sugar. Keep refrigerated.
