In a large bowl, whisk together flour, confectioners’ sugar and salt. Stir in melted butter to combine. Use your hands or the bottom of a measuring cup to press crust into bottom of lined pan in an even layer. Bake until crust starts to dry out and brown at edges, about 15 minutes.
While the crust bakes, prepare the filling: Combine lemon juice with starch. Wipe out bowl used for crust. In it, combine condensed milk, lemon zest and juice, and egg yolks. Whisk until thoroughly combined and homogenous. Filling will resemble pudding.
Once crust is baked, remove from oven but leave oven on. Using a fork, poke holes all over the top of the warm crust (not all the way through the crust)
Pour filling onto hot crust and smooth it in an even layer. Return pan to oven and bake until filling is set around edges but jiggles slightly in center when shaken, 20 minutes more. Remove from oven and let cool 1 hour. Then, transfer pan to refrigerator to chill completely, at least 1 hour.
To serve, grip parchment paper at edges to lift lemon square out of pan. Use a sharp knife to cut even squares. Sift confectioners’ sugar on top and serve chilled.