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 chocolate squares 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 square. (See Tip.) Place on cake; sprinkle with sugar. Keep refrigerated.
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