Butter one 9x13x2 baking pan and line with parchment paper.
Prepare the crust. Beat butter and sugar until smooth. In a small bowl, combine flour and salt. Add to butter mixture and beat at low speed until just combined.
Transfer the dough to the prepared pan and press evenly into the bottom of the pan. Use the back of a measuring cup for an even crust. Place the crust in the refrigerator for 30 minutes. Preheat oven to 350°F.
Remove the crust from the refrigerator and bake for 15-20 minutes, until slightly golden. Reduce oven temperature to 300°F.
While the crust is baking, prepare the filling. In a medium bowl whisk together flour and sugar. In a large bowl whisk together eggs, egg yolk, lemon juice, lemon zest, and salt. Combine both mixtures and pour over crust.
Bake bars for 25-30 minutes until center is set. Let bars cool completely. Cover with plastic wrap and refrigerate until chill. Using a sharp knife cut into 2x3-inch bars. Dust with powder sugar before serving.