In a large bowl, beat 2 cups cold heavy cream and 1/2 cup powdered sugar on medium-high speed until stiff peaks form, about 3-4 minutes. The cream should hold its shape when you lift the beaters. Transfer to a separate bowl and refrigerate.

In the same large bowl (no need to clean), beat softened cream cheese on medium speed until smooth, about 1 minute. Add pumpkin puree, maple syrup, vanilla, pumpkin pie spice, and salt. Beat until fully combined and smooth, scraping down the sides once.

Gently fold about one-third of the whipped cream into the pumpkin mixture to lighten it, then fold in the remaining whipped cream until no streaks remain. Use a rubber spatula and work from the bottom up to keep the mixture airy.

Spread a thin layer of pumpkin mousse on the bottom of a 9x13 inch dish. Arrange 9 graham crackers in a single layer, breaking pieces as needed to fill gaps. Spread one-third of the remaining mousse evenly over the crackers. Repeat with two more layers of crackers and mousse, ending with mousse on top.

Cover tightly with plastic wrap and refrigerate at least 8 hours or overnight. The crackers will soften and absorb moisture from the mousse, creating a cake-like texture.

Just before serving, beat 1 cup cold heavy cream and 2 tablespoons powdered sugar until soft peaks form. Spread or pipe over the chilled cake. Dust lightly with cinnamon. Slice with a sharp knife, wiping clean between cuts for neat slices.