For the lemon curd

1. Cook lemon curd. Whisk together the lemon zest, juice, sugar, and eggs in a medium saucepan. Add butter and cook over moderately low heat, whisking frequently, until curd is thick enough to hold whisk marks, about 6 minutes.
2. Strain and cool. Strain lemon curd through a fine-mesh sieve into a wide shallow dish, scraping bottom of sieve with a spatula into the dish. Cover the surface of the lemon curd with wax paper to prevent a skin from forming. Cool completely, stirring occasionally, about 30 minutes.

For the crust

1. Prep oven. Heat oven to 350 degrees F.
2. Form crust. Stir together the graham cracker crumbs, sugar, and salt. Add the butter and stir together until the butter is evenly mixed in. Press the crumb mixture onto the bottom and 1 inch up the side of a springform pan.
3. Bake. Bake the crust for 10 minutes, then cool crust completely in the pan on a rack.

For the filling and baking cheesecake

1. Prep oven. Reduce oven temperature to 300 degrees F.
2. Make filling. Beat together the cream cheese and sugar in a bowl with an electric mixer at medium speed until smooth, 1 to 2 minutes. Reduce speed to low and add eggs 1 at a time, beating until incorporated. Beat in sour cream and vanilla until combined.
3. Swirl. Pour 2/3 of cream cheese filling into cooled crust, then spoon 2/3 of lemon curd over filling and swirl curd into filling with a small knife. (Avoid touching crust with knife to prevent crumbs from getting into filling.) Repeat with remaining filling and curd.
4. Bake. Bake cheesecake until set 1 1/2 inches from edge but center jiggles slightly when pan is gently shaken, about 45 minutes. The center of the cake will appear very loose but will continue to set as it cools. Transfer springform pan to a rack and immediately run a knife around top edge of cake to loosen (this will help prevent cracks from forming as the cheesecake cools).
5. Cool and chill. Cool completely, about 2 hours, then chill, uncovered, at least 4 hours. Then cover cheesecake with plastic wrap until ready to serve. Remove side of springform pan before serving.