Directions

1. Butter an 8 inch square baking dish, and set aside. In a heavy saucepan, combine the sugar, sour cream, corn syrup and butter. Bring to a boil; cover and cook one minute. Stir until well blended. Remove the lid, and without stirring, heat to between 234 and 240 degrees F (112 to 116 degrees C), or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface.
2. Remove from the heat, and add the chocolate and vanilla. Do not stir. Set the pan over a wire rack until the mixture cools to about 175 degrees F (80 degrees C). This should take about 15 minutes.
3. Beat the fudge with a wooden spoon until it thickens and loses its gloss. Stir in walnuts, and pour into the buttered dish. Spread evenly, then set aside to cool. Cut into small squares.