Preheat oven to 350°F. Coat a 15" x 10-1/2" jelly roll pan with cooking spray. Set aside. Combine first 4 ingredients in large bowl. Beat at medium speed until smooth. Combine flour and next 6 ingredients in bowl, stir into pumpkin mix, and blend well. Stir in the walnuts. Spread batter in pan.
Bake for 25-30 min or until lightly browned. Cool completely. Make frosting while cake cools.
Beat cream cheese, butter and vanilla with mixer until creamy. Gradually beat in powdered sugar until smooth. Frost cooled cake.