In a mixing bowl, combine all-purpose flour, granulated sugar, and salt. Cut in unsalted butter until the mixture resembles coarse sand. Gradually add ice-cold water until the dough forms. Refrigerate for 1 hour, then roll it out to fit a 9-inch pie plate.

Preheat your oven to 425°F (220°C). Transfer the dough to the pie plate, pressing gently into the edges. Freeze for 30-60 minutes, then line with parchment paper and fill with pie weights. Bake for 17 minutes, remove the weights and parchment, and bake for an additional 5 minutes until lightly golden. Let it cool.

In a medium saucepan, whisk together granulated sugar, cornstarch, salt, lemon zest, and milk over medium heat. Cook while stirring until thickened, about 5-7 minutes. Remove from heat and let cool slightly before adding egg yolks.

In a small bowl, whisk together egg yolks. Slowly add a spoonful of hot filling to the yolks while whisking, then pour this back into the saucepan. Cook over low heat for an additional 2 minutes, stirring constantly until thickened. Stir in unsalted butter until smooth.

Remove the filling from heat and let cool for about 10 minutes. Gently fold in sour cream until well combined, then pour the filling into the cooled pie crust.

Cover the pie loosely with aluminum foil and refrigerate for at least 4 hours or overnight.

For the optional whipped cream topping, beat heavy cream with powdered sugar and vanilla extract until stiff peaks form.

Spread or pipe the whipped cream topping over the set pie. Add additional lemon zest or fresh berries if desired. Slice and serve chilled.