1. Prick each potato several times with a fork. Place potatoes in the microwave, and cook at HIGH 16 minutes or until tender, turning after 8 minutes. Let stand 2 minutes. Cut each potato in half lengthwise; scoop out flesh with a large spoon, and transfer to a bowl.
2. Combine milk and butter in a microwave-safe bowl, and microwave at HIGH 2 minutes or until butter melts. Add milk mixture to potatoes; mash with a potato masher to desired consistency. Stir in buttermilk and remaining ingredients.