Heat oven to 350°F. Grease and flour 13x9-inch baking pan; set aside.
Combine all cake ingredients except carrots and cinnamon in bowl. Beat as directed on cake mix package. Stir in carrots and cinnamon.
Spread batter evenly into prepared pan. Bake 35-40 minutes or until toothpick inserted in center comes out clean. Cool completely.
Combine all frosting ingredients in bowl. Beat at medium speed, scraping bowl often, until creamy. Spread cake with frosting.
One medium carrot will yield about 1 cup shredded carrot.