Preheat oven to 350 degrees. Line with parchment or foil and butter a 9x13-inch baking dish, set aside.
In a medium size mixing bowl whisk together the flour, cornstarch, powdered sugar, brown sugar and salt. Add cubed butter and blend (using a pastry blender) until the mixture begins to form pea-size crumbles. You can also do this in a food processor if you have one. Firmly press the mixture into an even layer on bottom and 1/2-inch up the sides of the prepared baking dish. Make sure you don't have any cracks in the crust. Place in the oven and bake until lightly golden brown, about 12-15 minutes.
Meanwhile, in a large mixing bowl, whisk together the granulated sugar, flour and salt. Slowly incorporate the lemon zest, lemon juice, eggs, egg yolks, heavy cream and vanilla (if using). Whisk vigorously until smooth (the mixture may be slightly foamy, that's ok). Pour over the hot crust, turn oven temperature down to 325 and return to oven to bake until the filling is set, about 20-25 minutes. Remove from the oven and allow to cool for 1 hour, then place uncovered in the refrigerator to chill for 2-3 hours before dusting with powdered sugar, cutting and serving.
Notes
-You can swap out 1/4 c. of lemon juice for orange juice, lime juice, strawberry or raspberry puree!