Place sugar in a saucepan; add cream and water, and stir until the sugar dissolves. Wipe down the sides of the pan with a wet cloth. Add a pinch of cream of tartar, place over medium heat and boil the mixture very slowly until it reaches the soft-ball stage — 240 degrees F on a candy thermometer. Add the butter and boil the mixture until it reaches the soft-crack stage — 280 degrees F on a candy thermometer. DO NOT STIR AFTER THE MIXTURE HAS COME TO A BOIL!
Remove the pan from the heat and add the vanilla extract. Pour mixture into a deep, buttered 7-inch square pan. When the butterscotch is nearly cold, use the point of a buttered or oiled knife to mark it into bars or squares. When the butterscotch is quite cold and set. Break it up, wrap each piece in wax paper, and keep the candy in an airtight container.