Combine sugar, cornstarch, lemon rind, juice, egg yolks and milk in a heavy saucepan and cook over medium heat, stirring constantly until thick. Stir in butter once thick and remove from heat and allow to cool to room temperature. Once it is room temperature stir in the sour cream and pour the filling into the pie shell.
With the heavy whipping cream, beat it in a chilled metal bowl on high speed until stiff peaks form, add to a pipping bag and pipe rosettes on top of pie and garnish with lemon twists. Place in refrigerator until chilled, and serve. I usually serve this with a little limoncello.