Preheat oven to 350 degrees. 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 to 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 to 30 minutes. Cool completely in pan.
Refrigerate until filling is firm, about 2 hours or up to 3 days. Using paper overhang, lift cake onto a work surface; cut into 16 squares, and dust with confectioners sugar.
