Place water in a small bowl and sprinkle gelatin over top. Whisk to combine and set aside.
In a medium bowl, beat egg yolks and sugar until thick and smooth, about 1 minute of beating. Heat milk in a medium saucepan over medium heat until steaming (do not let boil). Very slowly drizzle hot milk into egg yolk mixture, a little at a time to temper the egg yolks. Once all the milk has been added to egg yolks, add all back to saucepan. Cook over medium heat, stirring occasionally until mixture is slightly thickened and at a low simmer.
Offheat, strain hot milk mixture through a fine mesh sieve into a medium bowl. Whisk gelatin mixture, white chocolate, vanilla, and pumpkin pie spice into milk mixture until chocolate and gelatin is melted. Whisk in pumpkin puree and let cool several minutes.
In a separate medium bowl with a handheld electric mixer OR stand mixer, beat egg whites to stiff peaks. In a separate medium bowl, whip 1/2 cup heavy cream to stiff peaks.
Very gently fold beaten egg whites and heavy cream into cooled white chocolate mixture.
Spoon filling into cooled crust and smooth top. Refrigerate pie 3 hours until set.
When ready to serve, place remaining 1/2 cup heavy cream and powdered sugar in a large bowl or a stand mixer. Use a handheld electric mixer or a whip attachment to beat cream to stiff peaks.
Top pie with whipped cream and white chocolate shavings. Slice pie and serve. Enjoy!