01 - Whisk together pumpkin puree, heavy cream, sweetened condensed milk, whole milk, brown sugar, pumpkin pie spice, vanilla extract, and salt in a large mixing bowl until completely smooth.
02 - In a separate chilled bowl, whip heavy cream with powdered sugar and vanilla extract using an electric mixer or whisk until stiff peaks form.
03 - Arrange a single layer of graham crackers on the bottom of a 9x13-inch baking dish, breaking crackers as needed to fit.
04 - Spread one-third of the pumpkin mixture evenly over the crackers.
05 - Add one-third of the whipped cream over the pumpkin mixture and spread gently.
06 - Repeat layering with crackers, pumpkin mixture, and whipped cream two additional times, finishing with whipped cream on top.
07 - Smooth the top layer and dust with ground cinnamon or pumpkin pie spice, if desired.
08 - Cover the dish and refrigerate for at least 4 hours, or preferably overnight, to allow layers to soften and set.
09 - Cut into portions and serve chilled.