Microwave carrots and ¾ cup water in a glass bowl at HIGH 10 to 12 minutes or until tender.
Drain.
Process carrots and butter in a food processor until mixture is smooth, stopping to scrape down sides.
Transfer to a large bowl.
Process sweet potatoes until smooth, stopping to scrape down sides.
Add to carrot mixture.
Stir together sweet potato mixture, sour cream, and remaining ingredients.
Spoon into a 1½-quart glass dish.
(Cover and chill up to 2 days, if desired; let stand at room temperature 30 minutes.
) Microwave at HIGH 4 to 5 minutes or until thoroughly heated.
