Combine sugar, sour cream, and coffee in a medium saucepan. Cook over medium heat, stirring constantly, until sugar dissolves. Cover and continue to cook over medium heat 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, without stirring, until mixture reaches soft ball stage (234°).
Remove from heat; stir in vanilla and walnuts, mixing well. Stir just until mixture begins to cool. Pour mixture into a buttered 15- x 10- x 1-inch jellyroll pan. Quickly separate walnuts with a fork. Let cool, and store in an airtight container.