In a medium bowl, combine graham cracker crumbs, brown sugar, and melted butter until moist and crumbly. Press evenly into the bottom and up the sides of a 9-inch pie plate to form the crust. Chill in refrigerator while preparing filling.

In a large bowl, whisk together pumpkin puree, marshmallow fluff, pumpkin pie spice, cinnamon, vanilla, and salt until smooth.

In a separate chilled bowl, whip heavy cream and powdered sugar until stiff peaks form.

Gently fold the whipped cream into the pumpkin–marshmallow mixture until fully combined and light in texture.

Spoon the fluffy filling into the chilled crust, smoothing the top with an offset spatula.

Refrigerate for at least 4 hours, or up to overnight, to set properly.

Before serving, optionally dust the top with extra cinnamon or garnish with whipped cream and a sprinkle of pumpkin pie spice.