Combine sugar, cinnamon, cream of tartar, and water in a medium-size heavy saucepan; stir until well blended. Cook over medium heat, stirring occasionally, until mixture reaches soft ball stage (240°).
Remove from heat; stir in vanilla and walnuts. Beat with a wooden spoon until mixture begins to thicken. Quickly spread walnuts on waxed paper; separate with a fork. Let cool. Store in airtight containers.