Place 3 cups cantaloupe and next 3 ingredients in container of an electric blender; cover and process until smooth, stopping once to scrape down sides. Add remaining 3 cups cantaloupe; cover and process until smooth.
Combine yogurt and powdered sugar in a large bowl; stir in cantaloupe mixture. Cover and chill. To serve, ladle soup into individual bowls. Top each serving with 1 teaspoon sour cream. Garnish with mint sprigs, if desired.