1. In a small saucepan, combine 2 packets unflavored gelatin with 1 cup milk. Place over medium heat and whisk continually until the mixture just begins to steam and gelatin is dissolved then remove from heat and let cool 15 minutes (Do NOT boil).
2. In the bowl of an electric stand mixer on medium speed, whisk together 1 cup sugar, 16 oz sour cream, 15 oz pumpkin puree and 1 tsp pumpkin pie spice, and mix until blended. Add 16 oz of whipped topping and mix on medium low speed just until blended, scraping down the bowl as needed.
3. With the mixer on, slowly drizzle in the warm milk mixture and once it is fully incorporated, immediately divide between your ramekin serving cups or pour into an 11×13 glass casserole dish and smooth out the top. Refrigerate at least 4 hours or overnight for the mousse to set.