Preheat oven to 350°F. Spray pans with vegetable pan spray.
In large bowl, prepare cake batter as directed on package; stir carrots and, if desired, walnuts, into batter. Pour batter into prepared pans.
Refer to baking chart {link} for baking times and temperatures for specific pans. Cool 10 minutes in pan on rack; remove and cool completely before decorating.
YIELD: Makes about 5 1/2 cups batter