Preheat the oven to 325°F. Place an 8×8-inch pan on the bottom rack of the oven and fill halfway with boiling water.
Spray a 9-inch springform pan with nonstick cooking spray. Wrap the outside of the bottom of the pan with heavy duty aluminum foil, to prevent any leakage. Set aside.
Graham Cracker Crust: In a medium-size mixing bowl toss together graham crumbs with granulated sugar and melted butter until fully moistened. Press crumb mixture evenly onto the bottom and up the sides of the pan. Place the pan into the freezer to set while you make the filling.
Cheesecake Batter: In a large bowl of a stand mixer fitted with the paddle attachment, whip together cream cheese, granulated sugar and vanilla extract on low-medium speed for 2-3 minutes or until completely smooth. Add lemon zest and cornstarch, mix well. (Pro-Tip: Try not to beat too much air into the filling or the cheesecake could crack.)
Add the eggs one at a time, mixing well after each addition. Scrape sides and bottom of bowl as needed. Stir in sour cream by hand scraping sides of the bowl to fully incorporate with the cheesecake batter.
Pour cheesecake filling evenly into the crust. Place pan into the oven.
Bake on the middle rack of the oven for 1 hour 10 – 1 hour 20 minutes or until the center is still slightly loose when the pan is gently shaken. It won't be completely set.
Turn off the oven, open the oven door leaving slightly ajar. Allow the cheesecake to rest in the oven for 1 hour longer to gently finish baking.
Remove pan to a wire rack. Allow cheesecake to cool for 30 minutes then run a knife gently around the top edge. Let stand and cool completely to room temperature.
Spread top of the cheesecake with lemon curd, then leaving the ring in place, wrap and chill for at least 8 hours preferably 24 hours.
Meringue Topping: Preheat the oven to broil. Beat egg whites, vanilla and cream of tartar in a bowl with an electric mixer fitted with the whisk attachment on medium-high speed. Gradually add sugar, continuing to beat until stiff peaks form.
Mound whipped egg whites over top of lemon curd taking it all the way to the crust. Use an offset spatula to make fluffy swirls. Spread to the edge until the lemon curd is completely covered.
Transfer to the oven and broil for 3-4 minutes or until the meringue is lightly golden brown turning the pan for even browning. Don't walk away! (Alternately, use a kitchen torch to toast the meringue.)
Remove from the oven and chill for at least 1 hour, then slice and serve.