1In 2-quart saucepan, place beets. Add water to cover. Heat to boiling over medium-high heat. Cook 45 to 60 minutes or until beets are tender. Drain; cool. Peel and finely grate beets to equal 1 cup.
2Heat oven to 350°F. Spray 13x9-inch pan with cooking spray. In large bowl, mix flour, sugar, baking soda and salt. In medium bowl, beat eggs. Gradually add melted chocolate, oil and vanilla, stirring with whisk until blended. Add chocolate mixture to flour mixture, stirring with whisk until blended. Stir in sour cream; mix well. Stir in beets and carrots. Pour batter into pan.
3Bake 40 to 45 minutes or until toothpick inserted in center comes out clean. Cool completely in pan on cooling rack, about 1 hour.
4Frost cake with chocolate frosting; sprinkle with walnuts. Cut into 6 rows by 4 rows.