Pralines

Have ready 4 oiled large baking sheets. In a heavy 2-quart saucepan combine the sugars, the cream, the salt, and the cream of tartar, cook the mixture over moderate heat, stirring and washing down any sugar crystals clinging to the side with a brush dipped in cold water, until the sugar is dissolved, and boil it over moderately high heat, undisturbed, until a candy thermometer registers 238°F. Remove the pan from the heat, let the mixture cool until the candy thermometer registers 220°F., and stir in the butter and the vanilla. Beat the mixture until it is creamy and stir in the pecan halves. Working quickly, drop the mixture by tablespoon onto the baking sheets and let the pralines cool. Remove the pralines carefully and store them, wrapped individually in wax paper, in airtight container in a cool place. The pralines keep for 2 weeks.