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.