190g plain biscuit (digestives, milk arrowroot, etc.) (should be ~2 cups of crumbs)
1.5 Tbsp sugar
1/4 tsp. salt
85g unsalted butter, melted and slightly cooled
Filling
1 envelope unflavored gelatin (2½ tsp.)
1 tsp ground cinnamon
1/4 tsp ground nutmeg
1/2 cup sugar, scant
1/4 cup sugar, scant
1.5 Tbsp sugar
1/2 tsp salt
3/4 tsp salt
3 large egg yolks
3 large egg whites
3/4 cup whole milk
1¼ cups unsweetened pumpkin purée (about 300g)
3/4 cup heavy cream
1/4 cup sour cream
Method
Crust
Preheat oven to 160°C. Pulse biscuits (190g) in a food processor until broken down into fine crumbs (you should have about 2 cups). Set aside 2 Tbsp. biscuit crumbs for serving. Add sugar (1.5 Tbsp) and salt (1/4 tsp) and pulse just to combine. Add butter (85g) and pulse until mixture is the consistency of wet sand.
Transfer to a 9"-diameter deep pie dish. Press crumbs firmly onto bottom and up sides of dish. Bake crust until fragrant and edges just start to take on color, 20–25 minutes. Let cool.
Filling and assembly
Stir gelatin (1 envelope), cinnamon (1 tsp), nutmeg (1/4 tsp), sugar (1/2 cup), and salt (1/4 tsp) in a small saucepan. Whisk egg yolks (3) and milk (3/4 cup) in a small bowl to combine, then whisk into sugar mixture. Cook over medium heat, stirring frequently, until mixture begins to thicken and coats the back of a spoon (but do not let it boil), about 5 minutes. Stir in pumpkin purée (1 1/4 cup or 300g) and remove from heat. Transfer to a large bowl and chill until cool, about 10 minutes.
Meanwhile, using an electric mixer on medium-high speed, beat egg whites (3) in a large bowl until soft peaks form. With the motor running, gradually add sugar (1/4 cup) and continue to beat until stiff peaks form, 5–7 minutes.
Mix one-third of egg white mixture into chilled pumpkin mixture until smooth. Gently fold remaining egg white mixture into pumpkin mixture in 2 additions until incorporated, but don’t overmix.
Pour filling into biscuit crust; smooth top. Cover and chill overnight.
Vigorously whisk cream (3/4 cup) in a large bowl until medium peaks form. Fold in sour cream (1/4 cup) and remaining sugar (2 Tbsp) and salt (1/4 tsp) just to combine. If the cream looks like it could use another whisk or two to hold medium peaks, give it a whisk until it reaches the desired consistency. Using a large spoon, dollop a generous amount of whipped cream in the center of pie. Sprinkle with reserved graham cracker crumbs. Slice and serve with any remaining whipped cream alongside.