Add the mascarpone to a food processor along with the pumpkin purée, powdered erythritol, cinnamon, nutmeg, and vanilla extract. Blend until smooth and well combined. The mixture may be a little firm to start, but continue blending until soft and mousse-like. Take care not to over blend or the mixture may split. Please feel free to add more or fewer spices to taste.
Add the heavy cream to a large mixing bowl. Whisk the cream vigorously for several minutes until whipped and fluffy. The cream should be light, soft, and airy. Alternatively, you may whip the cream in the bowl of a stand mixer using a whisk attachment.
Carefully spoon the mascarpone and pumpkin mixture into the bowl with the whipped cream. Gently fold the whipped cream and pumpkin mixture together until completely combined and smooth. The mousse should be light and airy.
Divide the creamy pumpkin mousse evenly between four serving bowls or large ramekins. Transfer to the fridge to chill for 30 minutes or until ready to serve. Serve the mousse cold, optionally topped with low carb berries or a dusting of cinnamon.