Shortbread Base: Don't preheat oven yet. Line a 9 x 9-inch square baking dish with foil, Release foil or parchment, extending paper over edges of pan. Spray with cooking spray if using regular foil.
In a food processor, combine the flour, cornstarch, granulated sugar, salt and ground cinnamon. Pulse briefly until blended. Add the butter and pulse until the dough forms moist crumbs and sticks together when pinched, about 1 minute. Press the dough into the bottom and 1/2 inch up the sides of the prepared baking dish. Refrigerate for 30 minutes. Alternatively, you can skip the food processor and just cut all the ingredients together with a pastry cutter. Food processor makes it much easier.
Preheat oven to 350 degrees F. and bake the base until pale golden, 20 to 22 minutes.
Filling: Reduce oven temperature to 325°F. In a medium mixing bowl, whisk together the granulated sugar, flour, salt and lemon zest. Add the eggs, lemon juice and cream and whisk until just blended. Carefully pour mixture over the warm base. Bake until filling feels firm when touched lightly, about 20-22 minutes. Transfer pan to wire rack; cool to room temperature. Chill for 2 hours before cutting into squares or bars.
Store bars in an airtight container in refrigerator for up to one week. For the best flavor, remove from refrigerator and serve at room temperature. Do not freeze.