Spray 9-inch round pan with cooking spray; cover bottom with waxed paper. Microwave 4 oz. chocolate and butter in large microwaveable bowl on HIGH 1-1/2 to 2 min. or until chocolate is almost melted, stirring after 1 min. Stir until chocolate is completely melted. Stir in dry gelatin mix and sugar until well blended.
Add eggs; mix well. Stir in flour and nuts until well blended.
Pour into prepared pan.
Bake 40 min. or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool in pan 5 min. Run small knife around side of pan to loosen edge. Invert onto wire rack; remove waxed paper. Cool cake completely.
Meanwhile, melt 4 oz. of the remaining chocolate as directed on package; spread onto waxed paper-covered baking sheet. Refrigerate 30 min.
Microwave remaining chocolate in large microwaveable bowl as directed on package.
Whisk in COOL WHIP until well blended.
Transfer cake to plate; frost with COOL WHIP mixture.
Use sharp knife to cut chocolate on baking sheet into 1-1/2- to 2-inch irregular-shaped pieces. Arrange on cake. Top with raspberries and powdered sugar.
