Combine first 7 ingredients in a large saucepan and bring to a boil. Cover, reduce heat, and simmer 10 minutes or until vegetables are tender.
Place half of squash mixture in a blender. Remove center piece of blender lid (to allow steam to escape) and secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth.
Pour pureed mixture into a large bowl. Repeat procedure with remaining squash mixture. Return pureed mixture to pan over low heat. Add milk and butter to pan and cook 3 minutes or until thoroughly heated. Serve with yogurt and cilantro.