Heat oven to 350 F. Spray an 8-inch square baking pan with the cooking spray. Line the pan with 2 crisscrossed pieces of parchment, leaving an overhang on all sides; spray the parchment with the cooking spray.
Make the crust: In a food processor, combine the flour, butter, granulated sugar, and salt; pulse until fine crumbs form. Press the mixture firmly into the bottom of the prepared pan and bake until pale golden, 25 to 30 minutes.
Meanwhile, make the filling: In a medium bowl, whisk together the eggs, egg yolk, granulated sugar, lemon juice, lemon zest, flour, cream, and salt.
When the crust is baked, rewhisk the lemon mixture and pour onto the hot crust. Bake until just set, 15 to 18 minutes. Cool completely in the pan, then refrigerate until cold, about 2 hours.
Holding both sides of the paper overhang, lift the cake out of the pan, transfer to a cutting board, and cut into 24 rectangles (6 rows by 4 rows). Store the bars covered in the refrigerator for up to 3 days. Dust with the confectioners’ sugar before serving.