Bloom the gelatin by placing it in a bowl. Sprinkle the gelatin with the water and set aside for 5 to 8 minutes.
Meanwhile, place ¼ cup sugar, milk, spices, and salt in a saucepan. Cook over medium-high heat until the mixture barely comes to a simmer.
While the milk mixture is cooking, whisk together the egg yolks and remaining ¼ cup of sugar. Once hot, temper the milk mixture into the egg mixture by slowly whisking in a little of the hot milk in at a time.
Pour the egg-milk mixture back into the saucepan. Whisk in the pumpkin. Place the saucepan back on the stove over medium-low heat. Add the bloomed gelatin.
Stirring slowly yet constantly, bring the mixture to a slow boil (large bubbles will begin to “plop” on the surface). Continue whisking for another minute or 2 (or 160 degrees F) until thick and smooth.
Strain the pumpkin custard into a heat-safe container. Press a piece of plastic wrap directly on the top of the custard. Chill it in the refrigerator until completely cool.
After the custard cools, whip the 1 cup of cream. Begin mixing on medium-low speed until the cream begins to thicken slightly. Add the sugar and continue mixing on medium-high until soft peaks. Add the vanilla and mix until combined.
Fold the whipped cream into the cooled pumpkin custard. Start by stirring about ⅓ of the whipped cream into the pumpkin custard (no need to be gently here). Once the pumpkin custard is slightly lightened, fold in the remaining whipped cream.
Spoon the pumpkin chiffon filling into the pre-baked pie crust. Smooth it out with the back of a spoon or offset spatula. Return the pie back to the refrigerator to set, at least an hour or 2.
Top with a mound of fluffy whipped cream (or dollop small amounts on each slice when serving) and dust with cinnamon.