Microwave carrot and 3/4 cup water in a glass bowl at HIGH 10 to 12 minutes or until tender. Drain.
Process carrot 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 1 1/2-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.