1/2 cup (1 stick) unsalted butter, room temperature, plus more for pan
1/2 cup confectioners’ sugar, plus more for dusting
1/4 teaspoon salt
1 cup all-purpose flour, (spooned and leveled)
4 large egg yolks
1 can (14 ounces) sweetened condensed milk
3/4 cup fresh lime juice
Preheat oven to 350 degrees. Butter an 8-inch square baking pan. Line bottom with baking paper, leaving an overhang on two sides
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.
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.