PREPARATION:

1. Butter 8-inch square pan; set aside. Lightly butter side of heavy medium saucepan.
2. Combine sour cream, milk, corn syrup, butter and sugar in prepared saucepan. Cook over medium heat, stirring constantly, until sugar dissolves and mixture comes to a boil. Wash down side of pan with pastry brush frequently dipped in hot water to remove sugar crystals.
3. Add candy thermometer. Continue to cook until mixture reaches soft-ball stage (238°F).
4. Pour into large heatproof mixer bowl. Cool to lukewarm (about 110°F).
5. Add vanilla and beat with heavy-duty electric mixer until thick. Spread into prepared pan. Score fudge into small squares. Refrigerate until firm.
6. Cut into squares. Place walnut half on each piece. Refrigerate.