In a medium-sized saucepan, combine the cornstarch, sugar, lemon zest, lemon juice, egg yolks and milk. Stir well and heat over medium heat, stirring continuously.
After about 10 minutes, the mixture will start to thicken. Keep stirring until it begins to boil, and then remove the pan from the heat.
Stir in the butter until it's fully melted and combined.
Let the mixture cool for about 10 minutes and then stir in the sour cream.
Pour the lemon mixture into the prepared crust, and refrigerate it a few hours (or overnight) until it's completely cooled through. Store any leftovers in the refrigerator.