In a mixing bowl, combine the first five ingredients. Beat on medium speed for 2 mins; stir in walnuts. Pour into a greased 13x9 in baking pan. Bake at 350? for 45-50 mins or until a toothpick inserted near the center comes out clean. Cool on a wire rack. For frosting, in a mixing bowl, beat milk and pudding mix on low speed for 2 mins. Fold in the whipped topping. Spread over cake. Sprinkle with pistachios. Refrigerate for about 30 mins before cutting. Garnish with whole pistachios and mint if desired. Refrigerate leftovers.