1. Prepare graham cracker crust and let it cool completely.

2. Whisk sugar, cornstarch, and salt in a saucepan.

3. Gradually add water and cook over medium heat, stirring until thickened.

4. Beat egg yolks in a separate bowl.

5. Temper egg yolks with a small amount of hot mixture.

6. Return mixture to saucepan and cook until smooth and thick.

7. Remove from heat and stir in lemon juice, zest, butter, and vanilla.

8. Let mixture cool slightly, then fold in sour cream.

9. Pour filling into crust and smooth the top.

10. Refrigerate for at least 4 hours until fully set.

11. Top with whipped cream and berries before serving.