Stir the pumpkin puree and marshmallows together in a large bowl. Fill a smaller sauce pan with 2 inches of water and place the bowl on the pan. Turn the heat to medium low and bring to a low simmer.
Let the water remain at a low simmer while you stir the pumpkin-marshmallow mixture occasionally, until the marshmallows are melted and the pumpkin is combined.
Stir in the pumpkin pie spice and vanilla. Turn off the stove.
Carefully remove the bowl from the saucepan and let mixture cool. Once cooled, fold in the cool whip (or homemade whipped cream), reserving ½ cup for the topping.
Spread the pumpkin filling in the cracker crumb crust.
Place the remaining cool whip in a piping bag with star tip if desired (or a ziplock bag with one corner cut off). Pipe stars around the edge or spread the remaining cool whip over the top. Sprinkle with cinnamon.
Cover with a high-top lid and refrigerate for 2 hours or longer before serving.
Cut into 8 slices and enjoy.