To make lemon curd: Whisk together lemon juice, egg yolks and sugar in a heavy, nonreactive 1-quart saucepan. Cook over medium-low heat, stirring constantly with a heat-proof spatula or wooden spoon until mixture thickens and coats back of spoon, 5 to 10 minutes. (It should read 140° on a candy or instant-read thermometer.)
Remove saucepan from heat and strain mixture over a bowl. Stir in lemon zest and butter until butter completely melts. Cool slightly before using. Curd can be stored in refrigerator in an airtight container up to 1 week. Warm over a double boiler or microwave briefly to return it to liquid form. It will set up again when chilled.
To make crust: Preheat oven to 350° and position rack in middle of oven. Prepare a 9-inch springform pan by cutting a parchment circle to fit inside.
In a small bowl, combine graham cracker crumbs, sugar and salt. Using a fork, stir in butter until well combined. Press mixture into prepared pan. Bake 10 minutes. Cool on a wire rack while you prepare filling.
To make filling: Decrease oven temperature to 325°. In an electric stand mixer fitted with the paddle attachment, beat cream cheese at medium-low speed until creamy, about 2 minutes. Scrape down sides of bowl. With mixer on low, gradually add sugar, then flour and finally a pinch of salt. Scrape down sides again.
Switch to a whisk attachment and continue by mixing in vanilla, lemon juice and lemon zest. Whip in eggs and yolk, one at a time, scraping bowl and whisk at least twice. Continue to whip on low speed and add sour cream. Whip until well blended; do not overbeat. Batter should be light and somewhat airy.
Brush sides of springform pan with melted butter. Wrap outside of pan tightly in heavy-duty aluminum foil making sure there are no holes in foil. Pour filling into pan. Place pan in a large roasting pan and pour hot tap water into roaster so that it comes about halfway up springform pan. Transfer to oven and bake until filling is just set in center, about 45 minutes. If edges start to pull away from sides of pan sooner than that, remove it from oven. Immediately remove pan from water and remove foil. Place on wire rack and cool to room temperature.
Stir lemon curd until smooth (it should be the consistency of thick sour cream) and spread over surface of cheesecake. Cover springform with plastic wrap and refrigerate 8 hours or overnight. To unmold cake, place a hot towel around pan to loosen cake from sides. Run a butter knife around edges and remove outer part of pan.