In a small bowl, whisk the egg yolks. Pour about ½ cup of the hot milk mixture into the egg yolks and whisk thoroughly. Pour all of the egg mixture back into the milk mixture and cook, whisking constantly, until it begins to boil, about 3 minutes. Cook until the filling is thick and coats the back of a wooden spoon, 2 more minutes. Remove the pan from the heat and stir in the butter and lemon juice. Fold in the sour cream. Pour the filling into the prepared pie shell. Cover the filling with the meringue, making sure to spread it all the way to the edges of the crust.