Preheat oven to 350 degrees F. Combine 1 cup of the sugar, the lemon juice, grated rind, cornstarch, milk, and egg yolks. Cook over medium heat, stirring constantly, until thick. Let mixture cool completely. When cold, fold in sour cream and pour into cool pie shell. Beat egg whites with remaining ½ cup sugar until they form stiff peaks. Spread over filling and bake for 10 minutes or until top browns.