Mix the condensed milk, egg yolks and lemon juice together in a bowl and pour into the pie crust.With an electric mixer, beat the egg whites until foamy.
Add the cream of tartar and beat until soft peaks form. Gradually add the sugar and cornstarch and beat until peaks are stiff. Beat in the vanilla. Spoon meringue over the pie filling and bake at 325 degrees F. for about 25 minutes or until meringue is lightly browned.
Let the pie cool at room temperature for about an hour, then transfer to the refrigerator and chill for several hours.
