Lightly grease a 9-inch springform pan or line the bottom with parchment paper.
Combine the graham cracker crumbs, brown sugar, cinnamon, and melted butter. Mix until the crumbs are evenly moistened.
Press the crumb mixture firmly into the bottom of the prepared pan. Refrigerate while preparing the filling.
Sprinkle the gelatin over the cold water and let it stand for 5 minutes. Heat for 5 to 10 seconds, just until dissolved. Do not boil.
Melt the white chocolate in short microwave intervals, stirring between each one. Let it cool slightly.
Beat the softened cream cheese until smooth. Add vanilla and salt, then mix in the melted white chocolate and dissolved gelatin.
In a separate bowl, whip 1 cup heavy cream with 2 tablespoons powdered sugar until medium to stiff peaks form
Gently fold the whipped cream into the white chocolate mixture until smooth and fluffy
Spread the white chocolate mousse evenly over the chilled crust. Return the pan to the refrigerator.
To make the pumpkin mousse, beat mascarpone, pumpkin puree, powdered sugar, brown sugar, pumpkin pie spice, cinnamon, vanilla, and salt just until smooth.
In a clean bowl, whip 1 cup heavy cream until stiff peaks form
Gently fold the whipped cream into the pumpkin mixture until no white streaks remain.
Spoon the pumpkin mousse carefully over the white chocolate layer and smooth the top.
Cover and refrigerate for at least 6 hours, preferably overnight.
Before serving, whip 3/4 cup heavy cream with powdered sugar and vanilla until stiff peaks form.
Remove the cake from the springform pan and decorate with whipped cream and your preferred garnishes.
Slice with a warm, clean knife for the neatest layers.
