Lemon Lovers Pie

1 C sugar
3 T cornstarch
3 egg yolks
1/2  C lemon juice
1/2 tsp lemon extract
pinch salt
4 T butter
1 C milk
1 C sour cream

Mix sugar and cornstarch and add milk. Bring to a boil and cook several minutes and add egg yolks and boil gently. Next add lemon juice, lemon flavoring and a pinch of salt plus butter. Cool a bit and add one cup sour cream. Pour into a baked 9-inch pie shell, top with whipped topping. Refrigerate for at least one hour.