Three Cheese Mashed Potatoes

2 large potatoes, peeled and cubed
1/2 cup sour cream
1 (3 oz) package cream cheese, softened
2 tbs butter or margarine, softened
1/3 cup milk
1/4 cup (1 oz) shredded Cheddar cheese
1/4 cup (1 oz) shredded Muenster cheese
1 tsp salt
1/2 tsp pepper
1 tbs butter or margarine, cut up
Garnish:  minced fresh chives

Cook potato in boiling water to cover 15 min or until tender.  Drain.
Beat potato, sour cream, cream cheese, and 2 tbs butter at medium speed with an electric mixer until smooth.  Stir in milk and next 4 ingredients.  Spoon into a lightly greased 1 qt baking dish; dot with 1 tbs buter.  Cover and chill 8 hours, if desired; remove from refrigerator , and let stand 30 min.