Preheat the oven to 350-degrees.
For the ginger snap crust:
Toss the ginger snaps into a food processor, and process for about 20 to 30 seconds until finely ground.
Add the kosher salt and flour, process for about 10 to 15 seconds.
Pour in the melted butter and 1 tablespoon of the water. Pulse about 6 to 8 times to combine. Press the crumbs together between your fingertips – if they do not hold together add the additional 1 tablespoon of water and check again. Use the remaining water only if needed – too much will make it sticky.
Spray the bottom of 9” deep-dish pie plate. Turn the mixture into the pie plate and press in the bottom and up the sides. A measuring cup works well for this task.
Freeze the crust for 10 minutes. Place the pie plate on a baking sheet and bake for 15 minutes at 350-degrees. Set aside to cool completely while you make the filling.
For the pumpkin filling:
In a small bowl or measuring cup add the gelatin to the cold water, stirring to dissolve.
In a medium saucepan combine the brown sugar, kosher salt, cinnamon, ginger, allspice, pumpkin puree, egg yolks, and milk. Whisk together until the pumpkin mixture is smooth.
Cook over medium-low heat until the mixture comes to a boil, about 15 to 20 minutes, stirring occasionally. Boil for 1 minute and remove from the heat.
Add the dissolved gelatin to the pumpkin mixture, stirring to blend in completely and set aside to cool, about 20 minutes. While the pumpkin mixture cools make the meringue.
For the meringue:
Combine the egg whites and the cream of tarter in the bowl of a mixer. With the whisk attachment, beat the egg whites on medium-high until they are frothy, about 3 to 4 minutes.
Using medium speed, add the sugar a little at a time, scraping down the sides of the bowl once. Raise the speed to medium-high and beat until the meringue has formed soft peaks. Add the vanilla extract and continue beating on medium-high until the meringue is glossy and has formed stiff peaks, about 5 minutes.
To assemble the pie:
Gently fold the meringue into the cooled pumpkin custard, taking care not to deflate the meringue.
Pour the pumpkin filling into the cooled gingersnap crust. Chill at least two hours or overnight to allow the filling to set up.
Keep the pumpkin chiffon pie refrigerated.