Preheat the oven to 350°F. Spray a 12 cup Bundt pan with flour-added baking spray.
In a mixing bowl, combine cake mix, 3/4 cup water, 2/3 cup oil, 4 of the eggs and pudding mix.
Beat with an electric mixer on low speed until moistened. Beat on medium speed 2 minutes.
Prepare brownie mix batter according to package directions using 1/3 cup oil, 1/4 cup water and 1 egg. Reserve walnut packet for topping. Pour pumpkin batter into prepared pan. Top evenly with brownie batter.
Bake 55 to 60 minutes or until toothpick inserted in center comes out clean.
Let the cake cool for 30 minutes, then remove from pan to wire rack to cool completely.
Place frosting in small heavy-duty resealable plastic bag. Microwave on HIGH 10 seconds to soften. Knead until smooth. Cut small bottom corner off bag. Drizzle over top and halfway down sides of cake. Sprinkle immediately with reserved walnuts.