Position a rack in the upper third of the oven and set it to preheat at 350°F (175°C). Lightly grease a 9×9-inch baking dish, line with parchment paper ensuring some overhang for easy removal, and then grease the parchment itself.

To prepare the crust, beat the butter and granulated sugar together in a stand mixer fitted with a paddle attachment until the mixture becomes light and fluffy, approximately 3 to 5 minutes. Pause to scrape down the bowl, then add in the flour, salt, and poppy seeds. Mix at a low speed just until combined; the dough may hold together or remain slightly crumbly.

Press the crust mixture into the base of the prepared pan using your fingertips to form an even layer. Bake for 15 to 18 minutes, or until the edges begin to turn golden.

While the crust bakes, combine the eggs and sugar in a medium bowl, whisking until smooth and slightly thickened. Sift in the flour and cornstarch, blending them thoroughly. Add the lemon juice and zest, and whisk until the mixture is homogenous.

Carefully pour the prepared filling over the warm crust once removed from the oven. Return the pan to the oven and bake an additional 18 to 20 minutes, or until the surface is set and lightly golden, with no jiggle at the center.

Allow the bars to cool fully in the pan. Run a sharp knife around the edges, then lift the dessert from the pan using the parchment overhang. Cut into 9 or 12 squares as desired. Before serving, dust the tops with powdered sugar and a sprinkle of poppy seeds. For storage, place bars in an airtight container layered with wax paper.