Old-Fashioned Butterscotch

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