Preheat oven to 350 degrees. In a small bowl, stir graham cracker crumbs with sugar and melted butter until combined and moist. Press into the bottom and up the sides of a 9-inch springform pan. Bake 10 minutes and set aside to cool.
Reduce oven temperature to 325 degrees.
In a large bowl, stir cream cheese and sugar together until smooth and combined. Stir in vanilla and lemon extracts. Add in one egg at a time and stir in completely before adding in the next egg. Scrape edges and stir in lemon zest.
Add lemon curd into the cooked crust by dolloping it around the edges and into the center. (Using cold lemon curd with allow it to hold its shape while baking.
Pour prepared batter over top of the lemon curd and smooth the top. Bake 1 hour, then turn oven off completely, leaving cheesecake inside the oven to cool completely, about 6 hours.
Refrigerate cheesecake until completely chilled.
For the topping, using a hand mixer, whisk egg whites, granulated sugar and cream of tartar together in the top of a double boiler OR in a large, heat-safe bowl overtop of simmering water. Whisk 5-7 minutes or until topping is very thick, glossy and billowy. Fold in vanilla.
Spread over top of cheesecake and broil on low until topping has browned. OR use a kitchen torch to brown. Refrigerate until ready to serve.