For the Crust:
For the Filling:
Coat a 9-inch square baking pan with pan spray.
In a medium bowl, using an electric mixer on medium speed, mix the butter, cream cheese, shortening, and powdered sugar together for 5 to 6 minutes, until the mixture is completely smooth. (If using a standing mixer, use the paddle attachment.) Add the egg substitute and vanilla, and mix for another 2 minutes. Scrape down the sides of the bowl with a spatula. Then add the flour and mix for an additional 5 minutes.
Press the dough evenly into the prepared pan, covering the bottom and about ¼ inch up the sides. (If the dough becomes sticky, cover it with a piece of plastic film and continue to press with your fingertips or the flat bottom of a glass; remove the film before baking.) Place the pan in the freezer and chill for 10 to 15 minutes, until the dough is firm to the touch.
Meanwhile, preheat the oven to 350F with one rack in the upper third of the oven and another rack in the center position.
Prick the dough all over with a fork. Bake it on the center over rack for 20 minutes. Then move the pan to the upper rack, rotating the pan from front to back, and bake for another 10 minutes, or until the crust is a deep golden brown all over.
To make the filling: In a medium bowl, whisk together the lemon zest, lemon and orange juices, and egg substitute; set aside.
In a large bowl, whisk together the sugar, flour, and salt until well combined. Whisk in the juice mixture, then the eggs, continuing to whisk for 1 to 2 minutes longer, until the mixture is completely free of lumps.
To finish: Remove the crust from the oven, stir the filling once more, and then immediately pour it into the crust. Loosely drape a sheet of aluminum foil over the top and return the pan to the center rack of the oven. Bake for 35 to 40 minutes, until the filling is set.
Transfer the pan to a wire rack and leave it for several hours, until the pan is cool to the touch. Then refrigerate the pan for several hours for easiest cutting.
To serve: Cut into 4 strips in one direction, then 4 in the other to make 16 bars. Dust the tops of the bars with powdered sugar just before serving, if desired. Refrigerate any leftover bars, tightly wrapped, for up to 4 days, or freeze them for up to 2 weeks; thaw frozen bars overnight in the refrigerator before serving.
Source: Sweet & Skinny by Marisa Chruchill