In a food processor, combine pumpkin puree, cream cheese, light brown sugar, cinnamon, pumpkin pie spice, and nutmeg; process until smooth and thoroughly combined.
With machine running, add maple syrup, and continue to process until combined. Set aside.
In your mixer's bowl, beat 1 cup heavy cream on high until stiff peaks form, 2 to 3 minutes.
Using a rubber spatula, gently fold previously prepared pumpkin mixture into whipped cream.
Refrigerate until ready to use, or at least 30 minutes.
In the meantime, prepare the Whipped Cream.
In your mixer's bowl combine heavy cream, vanilla, and sugar.
Beat mixture until stiff peaks form.
Refrigerate until ready to use.
Assemble
Spread a little whipped cream on the bottom of a long and narrow plate or in a 9x13 pan.
Place one single layer of 4 whole graham crackers over whipped cream.
Spread 1/3 of the vanilla whipped cream over graham crackers.
Spread 1/3 of the pumpkin mousse over whipped cream.
Place a single layer of 4 whole graham crackers over pumpkin mousse.
Continue to layer until you've used up all ingredients, ending with whipped cream to cover the entire cake.
Cover the pan with plastic wrap and place in the freezer for at least 6 hours or overnight.
Top with chocolate shavings and chopped pecans.
Cut and Serve.