(From Everyday Food)
1/2 cup (1 stick) unsalted butter, room temperature, plus more for pan
1/2 cup confectioners sugar, plus more for dusting
1/4 t. salt
1 cup all purpose flour
4 large egg yolks
1 can (14 oz) sweetened condensed milk
3/4 cup fresh lemon juice (from about 3 lemons)
Preheat oven to 350. Butter an 8-inch square baking pan. Line bottom with parchment paper, leaving an overhang on two sides; butter paper.
Make crust: Using an electric mixer, beat butter, sugar and salt until light and fluffy. Add flour and mix on low just until combined. Press dough into the bottom and 1/2 inch up sides of prepared pan; prick all over with a fork. Bake until lightly golden, 15-20 minutes.
Make filling: In a large bowl, whisk together yolks, condensed milk, and lemon juice until smooth. Pour over hot crust in pan; return to oven and bake until filling is set, 25-30 minutes. Cool completely in pan.
Refrigerate until filling is firm, about two hours or up to three days. Using paper overhang, lift cake onto work surface; cut into 16 squares and dust with powdered sugar.