1. Add the flour, sugar and salt to the bowl of a food processor. Sprinkle the butter over the top and pulse the machine until coarse crumbles form.
2. Sprinkle the apple cider vinegar over and then, with the machine running, slowly pour in 3-4 tablespoons ice water. Stop the machine as soon as the dough begins to come together and turn the dough out onto a lightly floured surface. Knead briefly, then shape into two disks and wrap in plastic wrap. Place in fridge for at least one hour.
3. On a floured surface, roll out one disk of dough into a wide enough circle to cover your pie dish, dusting the top with small amounts of flour (if necessary) to prevent the dough from sticking to your rolling pin. Roll the pie dough up over your rolling pin in order to transfer it to your pie crust. Cut off excess dough and roll the edges in to create the crust.
4. In a large mixing bowl, stir together the pumpkin, egg yolks, spices, corn starch and sugar. Pour in the cream and stir until smooth.
5. In a separate bowl, add the egg whites. Use a hand mixer to whip to soft peaks, then gradually sprinkle in the remaining sugar and whip to stiff peaks.
6. Fold the egg whites into the pumpkin mixture until evenly combined and smooth. Pour into prepared piecrust.
7. If desired, roll out the second pie dough disk and use decorative stamps to punch out designs. Brush the crust and cut outs with beaten egg, then apply. Bake for 45 to 50 minutes, until the crust is brown, the filling is puffed up and set, and the middle barely jiggles. Remove from the oven and allow to cool.