Make the Pie Crust: In a large mixing bowl, whisk together flour, sugar, and salt. Cut in butter until mixture resembles coarse crumbs. Gradually add cold water, mixing until dough forms. Chill for at least 1 hour.

Blind Bake the Crust: Preheat oven to 425°F. Roll out crust to fit a 9-inch pie plate. Place in plate, trim excess, prick crust, line with parchment, and fill with weights. Bake for 17 minutes, remove weights, brush with egg wash, and bake for another 5 minutes until golden. Cool completely.

Prepare the Filling: In a mixing bowl, combine lemon zest and sugar. In a saucepan, whisk together the zest mixture, cornstarch, salt, lemon juice, and milk. Heat over medium until thickened. Temper egg yolks by whisking in a bit of the hot mixture, then add back to saucepan and cook for another minute. Stir in butter and let cool.

Fold in Sour Cream: Once filling has cooled slightly, fold in sour cream until combined. Pour filling into cooled pie crust, smoothing the top.

Chill the Pie: Cover pie with plastic wrap and refrigerate for at least 4 hours or overnight.

Serve with Whipped Cream: Whip heavy cream with powdered sugar until soft peaks form. Spread over chilled pie and garnish with lemon zest if desired.