For the Crust:
1/2 cup plus 3 tbsp. cold butter, cut into 1" pieces
1 1/2 cups all-purpose flour
1/2 cup confectioners' sugar
3 tbsp. cornstarch
1/2 tsp. salt
For the Topping:
2 cups granulated sugar
4 large eggs, lightly beaten
3 tbsp. all-purpose flour
1/2 cup + 2 tbsp. + 3/4 tsp. squeezed lemon juice
1/4 cup + t tbsp. milk
1/4 tsp. coarse salt
Preheat oven to 350F with a rack in the center of oven. Butter a 9 x 13 inch baking pan.
To make crust: In the bowl of a food processor, combine flour, confectioners' sugar, cornstarch and salt; process to combine. Add buter to processor and process until mixture is pale yellow and resembles a coarse mea., about 10 seconds. If you don't have a food processor, whisk together flour, confectioners' sugar, cornstarch, and salt in a large bowl. Add butter and work into flour mixture using a pastry blender or your fingers.
Pour crust mixture into prepared baking dish, pressing down with your fingers to creat a 1/4" thick layer along the bottom and 1/2" up the sides, pressing firmly at the edges to seal. Transfer pan to freezer and freeze for 30 minutes. Transfer to oven and bake, rotating pan once during baking, until golden brown, about 20 minutes.
Meanwhile, make the topping: In a large bowl, whisk together sugar, eggs, and flour; stir in lemon juice, (I used RealLemon), milk, and salt until well combined.
Remove baking pan from oven. Stir topping and pour into warm crust. Return pan to oven and continue baking until topping is just set but not browned, about 20 minutes.
Transfer baking pan to a wire rack to cool completely. Cut into 20 squares. Serve immediately or wrap each bar tightly with plastic wrap and keep refrigerated until ready to serve.