Move the oven rack to the lower third position and heat the oven to 350°F. Wrap the outer bottom of a 9-inch springform pan tightly with heavy-duty aluminum foil.
Combine the vanilla wafer crumbs, sugar, and melted butter. Press the crumb mixture into the bottom and about 2-inches up the sides of the pan. Bake for 10 minutes. Set aside to cool while you make the filling.
Make the Cheesecake
Reduce the oven temperature to 300ºF.
In a medium bowl, use your fingertips to combine the sugar and lemon zest, mixing until fragrant.
In a large mixing bowl beat the cream cheese with an electric mixer until smooth and creamy, about 1 minute. Add the lemon-sugar mixture, sour cream, lemon juice, and vanilla and beat on low speed until smooth. Using a silicone spatula, gently stir in the eggs just until combined. Pour the batter over the prepared crust.
Fill a roasting pan with a few inches of boiling water and place the cheesecake in the center. Bake for 1 hour, OR until the edges are set and the center is slightly jiggly. Turn the oven off, and leave the cheesecake in the water bath in the oven for another hour.
Make the topping
While the cheesecake is baking, make the curd. Add the egg, egg yolk, lemon zest, lemon juice, sugar, and cornstarch in a large bowl. Whisk together until well combined and cornstarch is fully dissolved.
Pour into a medium saucepan and cook over medium-low heat, stirring constantly with a wooden spoon, until thickened; about 7-10 minutes. You'll know it's ready when the curd coats the back of the spoon and is about the thickness of pudding.
When the curd has thickened, immediately remove it from the heat and stir in the butter.
Run the curd through a fine-mesh sieve to remove any bits of scrambled egg and transfer to a small bowl. Cover the bowl with plastic wrap making sure the wrap is touching the surface of the curd to prevent a skin from forming and refrigerate until ready to use.
Remove the cheesecake from the water bath and place it on a wire rack. Run a small knife around the outer edge of the cheesecake to loosen it from the pan and leave it to cool completely, about 2 hours
Spread the curd evenly over the top of the cheesecake. Cover and refrigerate for at least 4 hours, preferably overnight.
Remove the sides of the pan just before serving. Garnish with whipped cream and lemon slices.
Notes
Plan ahead: The recipe requires a lot of time. It's best to make it a day before you plan to serve it.
Room temperature ingredients: All of the cold ingredients should be brought to room temperature before using them in the cheesecake batter.
Don’t over mix: Keep the mixer on medium-low to low speed and use the paddle attachment. Otherwise, you could incorporate too much air into the batter which will cause the cheesecake to crack.
Don't over bake: The cheesecake should appear slightly underdone after baking. It will continue to set and firm up once cooled and refrigerated.
The cheesecake will keep for up to 4 days covered and stored in the refrigerator.
Or freeze the whole cheesecake. Cool and chill as directed. Wrap tightly in two layers of plastic wrap, then again in two layers of aluminum foil. Freeze for up to 3 months. Thaw in the refrigerator overnight.
You can also freeze individual slices. Wrap each slice in plastic wrap, then place them in a freezer-safe zip-top bag for up to 3 months. Thaw in the refrigerator overnight or at room temperature for about 30-45 minutes.