Lightly spray a 9-inch pie dish with non-stick cooking spray and set aside.
In the bowl of a food processor, pulse together the graham crackers for 30 seconds or until they are a medium to fine crumb.
Drizzle the melted butter and add in the sugar. Pulse again until the consistency is like wet sand.
Press the graham cracker mixture into the bottom and halfway up the sides of your prepared pie dish using slightly damp fingers or the bottom of a measuring cup.
Cover with plastic wrap and place the prepared crust in the freezer until you are finished with your mousse.
Combine the softened cream cheese and pumpkin puree to the bowl of a stand or hand mixer fitted with a whisk attachment. Mix well on low speed.
Add in the brown sugar, vanilla extract, and pumpkin pie spice. Mix on low until the mousse is smooth and incorporated.
Gently fold in 2 cups of the the whipped topping with a rubber spatula until light and fluffy. Keep the remaining 2 cups of whipped topping chilled until ready to finish.
Gather your pie dish from the freezer and add the mousse mixture.
Cover the mousse with plastic wrap and place in the refrigerator for 6 hours or overnight.
When you are ready to serve, spread the remaining whipped topping over the mousse and garnish with optional cinnamon garnish.
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