Beat the egg yolks with ½ C sugar until thick and lemon-colored. Add the pumpkin, milk, salt and spices and blend well. Turn into the top of a double boiler over medium boiling water and cook, stirring until thick and darkened, about 15 minutes.
During this time mix the gelatin and water and stir to blend out all lumps. When the pumpkin mixture is thoroughly warmed add the gelatin and mix well. Set aside to cool. Stir from time to time to help cool the mixture.
When thoroughly cooled, in a separate bowl beat the three egg whites with ½ C sugar on high speed until the egg whites are glossy and form stiff peaks when the beaters are turned off.
Starting with 1/3 of the egg whites, fold into the pumpkin mixture scraping the bottom of the pan and lifting up and over the top. Don’t stir. The scraping and lifting will preserve the air beaten into the egg whites. When the mixture is well blended, add another 1/3 of the egg white mixture and repeat the folding process until the egg whites are incorporated. Finish with the final 1/3 of the egg white mixture, scraping, lifting and folding over until the mixture of egg whites and pumpkin is well-blended without white streaks. Turn the mixture into the baked pie crust.
Refrigerate at least 5 hours, or overnight. Just before serving beat the whipping cream with the vanilla and the powdered sugar until the cream is thick and silky but not overbeaten so that it becomes lumpy. Spread the whipped cream on top of the pie in nice swirls, cut into large pieces and enjoy!