Crust:
Preheat oven to 350° and put rack in middle position.
Process cookies in a food processor until finely ground. Add butter and zest and pulse until combined. Press mixture into bottom of a 9" spring-form pan. Bake until lightly browned and set, about 10 minutes. Cool completely on a wire rack, at least 30 minutes.
Curd:
While crust is cooling, whisk egg, egg yolk, sugar and salt together in a small saucepan. Add lemon juice and cook over medium low heat, stirring constantly until thick and pudding like, about 3 minutes. Remove from heat and stir in butter and cream. Press through a fine mesh strainer into a small bowl and refrigerate until needed.
Filling:
Combine gelatin and lemon juice in a small bowl and let stand until gelatin softens, about 5 minutes. Microwave until mixture is bubbling around the edges and gelatin dissolves, about 30 seconds. Set aside.
With electric mixer on medium speed, beat cream cheese, sugar and salt until smooth and creamy, scraping down the sides of the bowl. Slowly add cream and beat until light and fluffy about 2 minutes. Add gelatin mixture and 1/4 C of the curd. Increase speed to medium high and beat until smooth and airy, about 3 minutes.
Pour filling into cooled crust and smooth top. Pour thin lines of remaining curd on top of cake. Lightly drag knife through lines to create a marbled effect. Refrigerate at least 6 hours. Remove sides of pan and serve. Cheese cake can be covered in plastic wrap and refrigerated for up to 3 days.
