Whisk sugar and cornstarch in a 1 quart saucepan to mix. Whisk in milk, egg yolks and lemon juice until smooth.
Bring to a boil over medium heat, stirring gently with a rubber spatula. Boil, stirring constantly, 30 seconds or until translucent and thick. Remove from heat. Add lemon peel and butter; stir until butter melts. Cover surface directly with wax paper to prevent a skin from forming. Cool to room temperature.
Fold in sour cream until well blended. Spoon about 1/3 cup into each crust. Cover loosely and refrigerate at least 4 hours until sest, or up to 2 days.
To serve, garnish with whipped cream and lemon slices.