Preheat the oven to 350°Grease a 9- × 9-inch baking pan.
Cut out a 9- × 9-inch square of parchment paper and lay on top of the greased bottom of the pan
For the crust, use a pastry blender to combine the flour, cornstarch, sugar, butter, and lemon zest until the mixture is coarse crumbs, with the butter worked in. Press the dough into the greased baking pan, on top of the parchment paper. Bake for 15 minutes, until lightly browned. Remove and place the pan on a wire rack to cool slightly. Leave the oven switched on.
For the cream cheese layer, fit a stand mixer fitted with the paddle attachment. Beat the cream cheese and sugar together until light and creamy, 3 to 4 minutes. Beat in the egg completely, then pour the mixture onto the warm crust. Return to the oven and bake for 10 to 15 minutes, until slightly set on top. You need to have the top cooked in a slight crust so the lemon layer doesn’t seep through when poured on top. Remove. Leave the oven switched on.
For the lemon layer, whisk together all the ingredients by hand. Gently pour over the cream cheese layer, trying not to disturb it. Spread out the lemon in an even layer.
Return to the hot oven and bake for another 25 to 30 minutes, until the filling is just set in the center.
Cool at room temperature, then refrigerate uncovered to letthe filling set completely. This should be refrigerated at least 3 hours, but more time may be needed.
Sprinkle with icing sugar if desired. Slice into squares to serve.
These can be stored in an airtight container for 2 to 3 days in the refrigerator, or frozen in a freezer-safe container for up to 3 months.
Nutritional values may vary.
You can double this recipe and bake in a 9x13 pan for even thicker bars that you serve as a "dessert slice" and not dainties.