In a large bowl, beat the cold heavy cream with an electric mixer on medium-high speed until soft peaks form, about 3-4 minutes. The cream should hold its shape but still curl at the tip when you lift the beaters. Do not overbeat or it will become grainy.

In a separate medium bowl, whisk together the pumpkin puree, powdered sugar, maple syrup, vanilla extract, cinnamon, ginger, nutmeg, and salt until completely smooth and no lumps remain, about 1 minute.

Take about one cup of the whipped cream and fold it vigorously into the pumpkin mixture to lighten it. This step is crucial - it prevents the remaining cream from deflating when you combine everything.

Add the lightened pumpkin mixture back to the bowl with the remaining whipped cream. Using a rubber spatula, fold with a gentle figure-eight motion, turning the bowl as you go, until no streaks remain and the mixture is a uniform pale orange. Work quickly but gently to maintain volume.

Divide the mousse among 6 serving glasses or ramekins, filling about three-quarters full. Cover loosely with plastic wrap and refrigerate for at least 2 hours, or up to 24 hours, until fully set and chilled through.

Top each mousse with a dollop of whipped cream and a sprinkle of crystallized ginger just before serving. The mousse should be spoonable and hold its shape on the plate.