Set out your cream cheese to soften at room temperature.
Make the lemon filling. Separate six eggs, placing the yolks in a small bowl. (Save the whites in a sealed plastic container and refrigerate or freeze them for another use.) Beat your egg yolks lightly with a fork and set them aside at room temperature, near your stove.
Rinse and dry four lemons. Zest the lemons until you have 4 teaspoons of zest. Cut your lemons in half and squeeze them until you have 2/3 cup of lemon juice. (You might need a 5th lemon.) Set the juice and zest next to your stove.
In a medium, heavy-bottomed saucepan (I use my stainless steel one) whisk together your sugar, flour, corn starch and salt. Gradually whisk in the water. Make sure your corn starch mixture is fully incorporated before placing the pan over heat. Use a silicone spatula for gently stirring from this point forward. Place the pan over medium-high heat and gently stir until thick and boiling. Reduce the heat. Cook and stir gently for two more minutes, and remove the pan from the heat.
To temper your egg yolks so they don't scramble, gradually mix in 1 cup (two standard ladles) of the hot filling into your bowl of yolks. Then, add the yolk mixture to your pot with the rest of the filling. Stir gently over medium-high heat and bring to a gentle boil. (Lower the heat if needed.) Cook and stir gently two minutes more. Remove from the heat. Your filling should be very thick.
Carefully stir in the butter and lemon zest. Gradually stir in the lemon juice (gently!), making sure the liquid is fully incorporated before adding more. Stir until there is no liquid pooling at the sides of your pan.
Pour the hot filling into a bowl or pan that fits in your freezer. Freeze the filling for 20 minutes, until it's firm to the touch and set. Cover the filling and transfer it to the refrigerator while you make the rest of the dessert.
When the filling is chilling in the freezer, make your crust. Preheat your oven to 350 degrees F. Add your flour, sugar and salt to a medium bowl and whisk them to combine.
Melt your butter, covered, in the microwave (use a bowl that's microwave safe.)
Pour the melted butter over your dry ingredients and use a fork to stir until everything comes together into a soft dough.
Thoroughly grease the bottom of a 9x13 glass pan (metal works too, but the glass pan will show off the layers.), using solid butter or baking spray. Transfer the crust dough to the pan and use clean hands to press it evenly into place along the bottom.
Bake the crust for 20 minutes, or until it's golden brown. Set the crust on a metal rack to cool.
While the crust is in the oven, make your cream cheese filling. Place your softened cream cheese and sugar in a medium bowl or stand mixer. Beat until blended and creamy. Refrigerate, covered, until you're ready to assemble your dessert.
To make the whipped cream, pour your whipping cream into a clean, medium stainless steel bowl. Add the vanilla and beat on low with clean beaters until frothy. Add your confectioners sugar and beat on medium, then high, until you have thick whipped cream. Cover and refrigerate until you're ready to assemble the dessert.
To assemble the lemon lush, make sure your crust is cooled off. Spread the cream cheese mixture over the crust in an even layer with a butter knife or silicone spatula.
Scoop the chilled lemon filling onto the cream cheese layer. Smooth out the lemon layer with a clean silicone spatula.
Spoon the whipped cream onto the lemon layer and spread it evenly with a clean spatula or butter knife.
Cover the dessert with plastic wrap or foil, without letting it touch the whipped cream. Refrigerate the lemon lush for several hours, but ideally overnight, so the layers can firm up and chill thoroughly.
Cut into slices and serve chilled. Refrigerate leftovers for 3-4 days.
It's best to make this dessert a day ahead, so it has time to set up and chill thoroughly. At the very least, make your lemon filling a day ahead. Then you could make the other layers early the next day, assemble the lemon lush, and refrigerate it for several hours before serving.