1. In a mixing bowl, combine the first 5 ingredients. Beat at medium speed for 2 minutes; stir in walnuts.
2. Pour into a greased 13 x 9 x 2-inch baking pan. Bake at 350 degrees F. for 45 to 50 minutes until a toothpick inserted near the center comes out clean. Cool on a wire rack.
3. Meanwhile, prepare frosting. In a mixing bowl, beat milk and pudding on low speed for 2 minutes. Fold in the whipped topping and spread over cake; sprinkle with pistachios. Refrigerate for about 30 minutes before cutting. Refrigerate leftovers.