Pre-bake the pie shell and set aside to cool.
Place brown sugar, gelatin, salt, cinnamon, nutmeg, ginger, and clove in a saucepan and mix with a whisk or a fork.
In a bowl, mix together the egg yolks, half and half or milk, and optional liqueur. Stir into the brown sugar mixture and let stand for a few minutes.
Turn the heat to medium, and stir until it comes to a boil. Remove from heat.
Stir in the pumpkin or sweet potato purée.
Place in fridge and let chill for about an hour.
Beat egg whites until soft peaks form. Rain the sugar into the egg whites, beating until stiff peaks form.
Remove the pumpkin or sweet potato mixture from the fridge and lightly fold in the egg whites, taking care to not to over mix. You may see some streaks in it but try to get it as homogenous looking as possible.
Turn the chilled mixture into the pre-baked pie shell. Return to fridge and let set for at least three hours. Four is even better.
Top with whipped cream or serve a la mode if you like.