Cook ingredients under low to medium heat, STIRRING MIXTURE CONSTANTLY until the filling thickens, this only takes about 10 minutes. The filling will THICKEN FIRST, then start to boil. As soon as it starts to boil, take it off the heat, add the butter. Stir until the butter is full incorporated into the filling. Let cool 10 minutes in the pan, add the sour cream. Stir until fully incorporated. Pour the filling into the baked pie shell. Refrigerate until firm, at least 3 hours. Garnish with whipped cream and serve.