2 cups granulated sugar
2/3 cup heavy cream
2/3 cup water
Pinch of cream of tartar
6 tablespoons butter, cut into small pieces
1/2 teaspoon vanilla extract

Place sugar in a saucepan; add cream and water, and stir until the sugar dissolves. 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.

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.