9 whole graham crackers
3 tablespoons sugar
1/8 teaspoon salt
5 tablespoons unsalted butter, melted
1 teaspoon unflavored gelatin
4 tablespoons water
5 large eggs (2 whole, 3 separated)
1 1/4 cups (8 3/4 ounces) sugar
1 tablespoon cornstarch
1/8 teaspoon salt
1 tablespoon grated lemon zest plus 3/4 cup juice (4 lemons)
1/4 cup heavy cream
4 ounces cream cheese, cut into 1/2-inch pieces, softened
FOR THE CRUST: Adjust oven rack to lower-middle position and heat oven to 325 degrees. Process graham crackers in food processor until finely ground, about 30 seconds (you should have about 11/4 cups crumbs). Add sugar and salt and pulse to combine. Add melted butter and pulse until mixture resembles wet sand.
Transfer crumbs to 9-inch pie plate. Press crumbs evenly into bottom and up sides of plate. Bake until crust is lightly browned, 15 to 18 minutes. Allow crust to cool completely.
FOR THE FILLING: Sprinkle 1/2 teaspoon gelatin over 2 tablespoons water in small bowl and let sit until gelatin softens, about 5 minutes. Repeat with second small bowl, remaining 1/2 teaspoon gelatin, and remaining 2 tablespoons water.
Whisk 2 eggs and 3 yolks together in medium saucepan until thoroughly combined. Whisk in 1 cup sugar, cornstarch, and salt until well combined. Whisk in lemon zest and juice and heavy cream. Cook over medium-low heat, stirring constantly, until thickened and slightly translucent, 4 to 5 minutes (mixture should register 170 degrees). Stir in 1 water-gelatin mixture until dissolved. Remove pan from heat and let stand for 2 minutes.
Remove 11/4 cups curd from pan and pour through fine-mesh strainer set in bowl. Transfer strained curd to prepared pie shell (do not wash out strainer or bowl). Place filled pie shell in freezer. Add remaining water-gelatin mixture and cream cheese to remaining curd in pan and whisk to combine. (If cream cheese does not melt, briefly return pan to low heat.) Pour through strainer into now-empty bowl.
Using stand mixer, whip 3 egg whites on medium-low speed until foamy, about 2 minutes. Increase speed to medium-high and slowly add remaining 1/4 cup sugar. Continue whipping until whites are stiff and glossy, about 4 minutes. Add curd–cream cheese mixture and whip on medium speed until few streaks remain, about 30 seconds. Remove bowl from mixer and, using spatula, scrape sides of bowl and stir mixture until no streaks remain. Remove pie shell from freezer and carefully pour chiffon over curd, allowing chiffon to mound slightly in center. Refrigerate for at least 4 hours or up to 2 days before serving.