Place the finely grated lemon zest and granulated sugar in a non-metal saucepan. Rub the zest into the sugar with your fingertips until the mixture becomes fragrant and the citrus oils have been released.
Beat the eggs in a separate small bowl, then set them aside. Position a fine-mesh strainer over a clean medium bowl.
Add the lemon juice, beaten eggs, and salt to the zest-infused sugar. Whisk until the ingredients are thoroughly blended.
Set the saucepan over low heat and cook for approximately 10–14 minutes, whisking continuously. Continue until the curd reaches 175°F and is thick enough to leave a coating on the back of a spoon.
Remove the saucepan from the heat. Pour the curd through the prepared strainer to eliminate any cooked egg particles or lumps.
Immediately add the vanilla extract and cubed salted butter. Whisk until the butter has melted completely and the curd is smooth.
Press plastic wrap directly against the surface to prevent a skin from forming. Refrigerate for at least 3 hours before using. The curd may be prepared up to 4 days in advance.
Preheat the oven to 350°F. Coat a 9-inch springform pan with nonstick cooking spray and confirm that the removable base is securely locked in place.
Combine the graham cracker crumbs with the melted salted butter. Stir until the crumbs are evenly moistened and resemble damp sand.
Transfer the mixture to the prepared pan. Compress it firmly across the base and extend it slightly up the sides.
Bake for 10 minutes. Remove the crust from the oven and allow it to cool.
After the pan has cooled, wrap the exterior base with foil in preparation for the water bath.
Ensure that the cream cheese, eggs, and sour cream have reached room temperature before mixing.
Combine the lemon zest and granulated sugar in a small bowl. Massage the zest into the sugar until the mixture becomes aromatic and slightly sandy in texture.
Beat the softened cream cheese in a large mixing bowl for 1–2 minutes, or until creamy and uniform.
Add the lemon-sugar mixture and continue beating for another 1–2 minutes. Stop periodically to scrape the bottom and sides of the bowl.
Incorporate the eggs one at a time at medium speed. Scrape the bowl after each addition and avoid excessive mixing.
Add the vanilla extract, sour cream, and lemon juice. Mix only until the filling is evenly combined.
Transfer 2½ cups of the plain cheesecake batter to a medium bowl. Gently fold 1 cup of chilled lemon curd into this portion.
Alternate spoonfuls of the plain batter and lemon-curd batter over the baked crust. Draw a butter knife through the two mixtures to create a marbled pattern without fully blending them.
Bring a large pot of water to a boil.
Place the filled springform pan inside a 10-inch cake pan. Position both pans inside a large roasting pan.
Set the roasting pan on the oven rack. Carefully pour boiling water into the roasting pan so that it surrounds the outer cake pan without entering it.
Bake at 350°F for 1 hour 40 minutes to 2 hours 12 minutes. The outer portion should appear set, while the center should retain a gentle wobble.
Turn off the oven and leave the cheesecake inside. Prop the oven door slightly open and allow the cheesecake to cool gradually for 1 hour.
Carefully remove the pans from the oven. Lift out the springform pan and discard any exterior foil.
Set the cheesecake on a wire rack and allow it to cool completely at room temperature.
Cover the cooled pan with foil and refrigerate overnight. The chilled cheesecake may be stored in the pan for up to 2 days before adding the topping.
Before preparing the meringue, wipe the mixing bowl and utensils with vinegar to remove any traces of grease.
Release the chilled cheesecake from the springform pan and transfer it to a serving plate. Spread a thin layer of lemon curd evenly across the surface.
Combine the egg whites, granulated sugar, cream of tartar, and salt in a clean glass or metal bowl.
Set the bowl over a saucepan containing approximately 1 inch of gently simmering water. Make certain that the base of the bowl does not touch the water.
Whisk continuously for approximately 8 minutes, or until the mixture registers 175°F. The sugar should be fully dissolved and the egg whites gently heated.
Remove the bowl from the saucepan and add the vanilla bean paste or vanilla extract.
Beat the mixture on high speed for 4–5 minutes, until the meringue is glossy and forms stiff peaks.
Immediately spoon the meringue over the cheesecake. Shape decorative peaks and swirls with the back of a spoon.
Carefully toast the surface with a kitchen torch, following the torch manufacturer’s safety instructions.
Slice and serve the cheesecake promptly after the meringue has been toasted. For the neatest portions, cut the dessert while it is thoroughly chilled.
Transfer leftovers to an airtight container and refrigerate for 3–4 days.