Prep: 20 minutes; Cook: 40 minutes.
Prepare Basic Cookie Dough, adding lemon zest at the end of step 3 (with egg white and vanilla). Press dough evenly into the bottom of a pan with a spatula, pressing down at the edges so dough doesn't form a rim after baking. Freeze dough for 15 minutes. Prick holes all over to keep it flat during baking. Bake for 25 minutes or until golden brown.
While cookie is baking, prepare lemon curd. Have ready a strainer, suspended over a medium bowl. In a heavy saucepan, stir together egg yolks and 3/4 cup sugar until well blended. Stir in butter, fresh lemon juice, and salt. Cook over medium-low heat, stirring constantly and scraping sides of pan, until thickened—don't let it come to a boil, or it will curdle. When steam appears, remove the pan briefly from heat, stirring constantly to keep mixture from boiling. When the curd has thickened, about 5 minutes, pour into strainer. Press with a spatula until only the coarse residue remains in the strainer; discard residue.
When cookie is baked, remove it from the oven, lower the temperature to 300°, and pour lemon curd on top. Spread it almost to the edge and return it to the oven for 15 minutes, or just until it starts bubbling around the edges. Remove from the oven and cool completely in the pan on a rack. Refrigerate for 1 hour to set curd before cutting into bars. Just before serving, sprinkle powdered sugar over the tops of the bars. Store the bars in an airtight container for 3 days at room temperature, for 3 weeks in the refrigerator, or for 3 months in the freezer.