Step 1

FOR THE CRUST: Adjust oven rack to middle position and heat oven to 350 degrees. Make foil sling for 8-inch square baking pan by folding 2 long sheets of aluminum foil so each is 8 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan.

Step 2

Whisk flour, sugar, and salt together in bowl. Add melted butter and stir until combined. Transfer mixture to prepared pan and press into even layer over entire bottom of pan (do not wash bowl). Bake crust until dark golden brown, 19 to 24 minutes, rotating pan halfway through baking.

Step 3

FOR THE FILLING: While crust bakes, whisk sugar, flour, cream of tartar, and salt together in now-empty bowl. Whisk in eggs and yolks until no streaks of egg remain. Whisk in lemon zest and juice. Transfer mixture to saucepan and cook over medium-low heat, stirring constantly, until mixture thickens and registers 160 degrees, 5 to 8 minutes. Off heat, stir in butter. Strain filling through fine-mesh strainer set over bowl.

Step 4

Pour filling over hot crust and tilt pan to spread evenly. Bake until filling is set and barely jiggles when pan is shaken, 8 to 12 minutes. (Filling around perimeter of pan may be slightly raised.) Let bars cool completely, at least 1½ hours. Using foil overhang, lift bars out of pan and transfer to cutting board. Cut into bars, wiping knife clean between cuts as necessary. Before serving, dust bars with confectioners' sugar, if using.