For the crust: Lightly brush a 9- by 13-inch baking pan with butter. Beat the butter and granulated sugar with an electric mixer in a large bowl until light and fluffy, about 4 minutes. Whisk the flour, cornstarch and salt together in a medium bowl. Slowly add the dry ingredients to the butter and mix just until combined. Evenly press the dough into the prepared pan. Cover and refrigerate at least 1 hour.
Preheat the oven to 325 degrees F. Pierce the dough all over with a fork. Bake until golden, about 30 minutes. Cool.
For the filling: Whisk the sweetened condensed milk, granulated sugar, flour, salt, eggs and yolks in a large bowl until thick and light. Lightly whisk in the citrus juices. Pour the filling into the crust. Bake until set but still loose in the center, about 25 minutes. Cool to room temperature, and then refrigerate at least 1 hour.
Cut into 2-inch squares and just before serving, dust generously with confectioners' sugar.