Preheat the oven to 325 ℉. Lightly butter a 9" deep dish pie plate.

In a large bowl, stir together the crushed gingersnaps, sugar, and butter. Press the mixture into the pie plate with the back of a spoon, making sure to cover the bottom and sides all the way up to the rim. Bake the crust for 10 minutes. Remove from the oven and cool on a baking rack.

In a large, heavy saucepan, stir together the gelatin, brown sugar, salt, nutmeg, ginger, cinnamon, milk, and 3 egg yolks. Turn the heat to a medium-low and stir continuously as you heat the mixture. Keep stirring until it begins to thicken and then move it from the heat and allow it to cool completely.

NOTE: Do not let the mixture boil because then the eggs will "cook and curdle". if this happens, you can strain out the egg bits but it's actually easier to begin all over again with fresh ingredients.

Stir the pumpkin puree into the cooled egg mixture. Then stir in the optional rum. Cover and refrigerate the mixture for about 2 hours, or until it turns into a gloppy pudding that is still loose but forms big globs or mounds.

Meanwhile, beat the egg whites with an electric mixture until white and frothy. Slowly beat in the white sugar until it forms stiff peaks. One glob at a time, fold the meringue into the pumpkin puree pudding-batter. Gently keep folding until the mixture is well blended. Pour the batter into the pie crust and refrigerate for at least 3 hours to allow the pie to set. Serve with whipped cream and crushed gingersnaps for garnish.