HEAT oven to 350°F. Coat 9-inch springform pan with no-stick cooking spray. Prepare brownie batter according to package directions using oil, water, egg and walnut packet. Spread batter in prepared pan. Bake 28 to 33 minutes or until center is set. Cool completely on wire rack.
SPRINKLE gelatin over cold water in small microwave-safe bowl. Let stand 2 minutes to soften. Microwave 30 seconds. Stir until gelatin is completely dissolved. Cool slightly.
MIX sugar and cocoa in large bowl. Add whipping cream and vanilla. Beat with electric mixer on medium speed until stiff. Add gelatin mixture. Beat until blended. Spoon over brownie layer, spreading evenly. Chill 2 hours. Remove sides of pan.
SPRINKLE sides of torte with finely chopped walnuts. Place fudge topping in small heavy-duty resealable plastic bag. Seal. Microwave on HIGH 10 seconds to soften. Knead. Cut a very small corner off bottom of bag. Drizzle evenly over chocolate mousse. Garnish with walnut halves. Chill.