Combine first 6 ingredients in a small bowl. Mash with a fork until blended. Cover and refrigerate.
Place potatoes in a large saucepan; cover with water. Bring to a boil. Reduce heat, and simmer 20 minutes or until tender; drain. Return potatoes to pan. Add buttermilk, salt, and pepper; beat with a mixer at medium speed until smooth. Serve with butter mixture.