This candy is a lesson in learning what the soft ball stage is! If cooked past the soft ball stage (236 degrees) it will become hard and crumbly. But get it just right, and WOW, a tangy taste on fudge.
The soft ball stage means, that when you drop a bit of the syrup into a cup of cold water, it tends to hold its shape (does not disintegrate). But you can't really pick it up and make a ball with it. Use a good candy thermometer until you get used to how it looks.
Butter a loaf pan and set aside. In a heavy-bottomed 2-quart saucepan, combine sour cream and sugar over medium heat. Bring to a boil, and cook to the soft ball stage (236 degrees, see note). Stir with wood or bamboo spoon, making sure it does not stick or burn. Do not cook past the soft ball stage.
Let sit in a pan of cold water for about 5 minutes to cool slightly, then add butter and beat until it loses its gloss. Stir in walnuts.
Spread and press the candy into the greased loaf pan, let cool, and cut into squares.