1. In a medium saucepan, whisk together corn starch and heavy cream until completely smooth. Add sugar and whisk again. Add lime juice and lime zest and whisk again. Add butter and bring mixture to boil over medium-high heat, stirring frequently. Boil for about 1 minute, stirring constantly. Reduce heat to medium-low and simmer for 5 minutes, until mixture is thick. Cool to room temperature. Fold in sour cream.
2. For topping, in a small bowl, beat whipping cream for 2 minutes on high. Add sugar and vanilla; beat again until firm. Fold in sour cream.
3. Pour lime filling into fully-cooked and fully-cooled pie crust. Spread whipped cream topping over lime filling. Cover carefully with plastic wrap or a cake dome, and refrigerate 4 hours before serving. Garnish with a thin slice of lime if desired.

Make-Ahead Tips:

• The filing can be made two days ahead. Cool lime filling to room temp, fold in sour cream, and store in an air-tight container in the refrigerator.
• The whipped cream topping can be made one day ahead. Store in an air-tight container in the refrigerator.
• The crust can be made one day ahead. Cool completely and wrap in plastic overnight.
• The entire pie can be made and assembled up to 24 hours in advance. Carefully cover with plastic wrap or a cake dome and store in the refrigerator.