Mix sugar and cornstarch in a medium-sized saucepan. Whisk in milk until smooth, then yolks until blended. Stir in lemon peel and juice.
Add butter and whisk constantly over medium heat 5-7 minutes, just until boiling. Remove from heat and stir 1 minute longer. Let cool to room temperature.
Stir in sour cream until well blended. Pour into cooled pie shell. Cover loosely and refrigerate at least 6 hours until set, or up to 2 days.
Shortly before serving, garnish with whipped cream/topping.
400 degrees 8-10 min