Combine yolks, sugar and lime juice in a stainless steel bowl. Mix with a wire whisk. Place bowl over a saucepan of boiling water and whisk constantly until thickened. Remove bowl and mix in the condensed milk. Pour into baked graham cracker pie shell.
Optional topping: Beat four egg whites; blend in 6 tbls. sugar and ���½ tsp. cream of tartar. Beat until stiff and peaks form. Cover filling with topping and bake in 350-degree oven until egg whites are golden brown