Spray a 9-inch square baking pan with nonstick cooking spray. Fold two 16-inch pieces of foil lengthwise to measure 9 inches wide. Fit one sheet in the bottom of the greased pan, pushing it into the corners and up the sides of the pan (the overhang will help in removal of the baked bars). Fit second sheet in the pan in the same manner, perpendicular to the first sheet. Spray the sheets with nonstick cooking spray.
Place flour, confectioners' sugar, and salt in a food processor and process briefly. Add the butter and process to blend, 8 to 10 seconds, then process until the mixture is pale yellow. Sprinkle mixture into prepared pan and press firmly with your fingers into an even layer over the entire pan bottom. Refrigerate for 30 minutes.
Adjust oven rack to the middle position and heat oven to 350 degrees. Bake crust until golden brown, about 20 minutes.
In a medium nonreactive bowl, whisk together the yolks and whole eggs until combined. Add granulated sugar and whisk until just combined. Add lemon juice, zest and salt; whisk until combined, about 5 seconds. Transfer mixture to a medium nonreactive saucepan, add the butter pieces, and cook over medium-low heat, stirring constantly with a wooden spoon, until curd thickens to a thin sauce-like consistency and registers 170 degrees on an instead-read thermometer, about 5 minutes. Pour curd immediately through a single-mesh stainless steel strainer set over a clean nonreactive bowl. Stir in the heavy cream; pour curd onto the warm crust immediately.
Bake until the filling is shiny and opaque and the center 3 inches jiggle slightly when shaken, 10 to 15 minutes. Cool on a wire rack to room temperature, about 45 minutes. Remove the bars from the pan using the foil handles and transfer to a cutting board. Cut into 2 1/4-inch squares, wiping the knife clean between cuts as necessary. Sieve confectioners' sugar over the bars, if desired.