1. Spray 9-inch round pan with cooking spray; cover bottom with parchment.
2. Microwave butter and 4 oz. chocolate in large microwaveable bowl on HIGH 1-1/2 to 2 min. or until butter is melted, stirring after 1 min. Stir until chocolate is completely melted and mixture is well blended. Add sugar; mix well. Blend in eggs and vanilla. Add flour; mix well. Stir in nuts. Pour into prepared pan.
3. Bake 40 min. or until toothpick inserted in centre comes out with fudgy crumbs. (Do not overbake.) Cool in pan 10 min. Loosen torte from side of pan with knife. Invert torte onto wire rack; gently remove pan and parchment. Cool torte completely.
4. Melt remaining chocolate as directed on package; drizzle over torte. Let stand 15 min. or until chocolate glaze is firm.