key lime pie

Here's another place where she has the recipe and it says where to put the sour cream http://www.almacucina.com/2013/05/key-lime-pie.html Here's what it says:
"In a medium, heavy bottomed pot whisk together 4 tablespoons heavy cream (reserve the 1/2 cup for later), sugar, lemon juice, lime juice and corn starch. Cook on medium heat, stirring often until mixture thickens, about 5 minutes. Remove from heat and fold in sour cream and lime rind."