Preheat your oven to 350°F and line a small baking sheet with parchment paper. Spread the pumpkin puree evenly across the sheet using a spatula, then bake it for 25-30 minutes until it darkens slightly and firms up. Let it cool completely before moving on.
While the pumpkin cools, add the heavy cream and powdered sugar to a mixing bowl. Beat on medium-high speed until it thickens, then increase to high until it forms soft, billowy peaks. Set the whipped cream aside until you're ready to use it.
Once your roasted pumpkin has cooled, scrape it into a small bowl and stir in the pumpkin pie spice and vanilla extract. Mix until it's completely smooth, since this step also helps break up any lumps that formed during roasting.
Add the spiced pumpkin to your whipped cream and gently fold with a spatula until there are no more streaks of white or orange. Take your time here so you don't knock all the air out of the whipped cream.
Portion the mousse into dessert cups right away, or refrigerate it briefly for a slightly firmer texture. Top with a dollop of whipped cream and a light dusting of pumpkin pie spice just before serving.