Preheat oven to 350. Prepare a 9" x 13" pan with cooking spray; set aside. In a mixing bowl, combine cake mix, whipped topping mix, carrots, walnuts, raisins, cinnamon, and baking powder. In another mixing bowl, combine water, egg whites, baby food, oil, and one teaspoon vanilla extract. Mix dry ingredients with wet ingredients just until moistened. Spread mixture into prepared pan. Bake for 40 minutes, or until toothpick inserted in center comes out clean. Meanwhile, beat cream cheese until smooth. Add lemon peel, lemon juice, and remaining vanilla extract. Gradually add powdered sugar one cup at a time. Mix until smooth. Spread over cooled cake. Sprinkle top with remaining half cup walnuts. Refrigerate two hours before serving.