Dice all but 3 of the strawberries, which will be used for toppings later. Add the diced strawberries and sugar to a medium pot. Place the pot over medium-low heat and stir occasionally as the strawberries cook down for 10 minutes.
Stir in the cornstarch and cook the sauce for 2 more minutes to create a thick mixture. Take the pot off the heat, stir in the vanilla, and let the sauce cool for 10 minutes.
Purée the strawberries with a hand mixer, leaving a few small chunks of strawberries. (Or use a blender if you want a finer consistency.) Set aside 1/2 cup of this strawberry sauce to use for the bottom fruit layer of the servings later.
Beat the cream cheese into the strawberry sauce with the hand mixer. Beat it for a full 3 minutes, as this will help fluff the mousse up by incorporating air into it.
Then, gently fold the whipped topping into the mousse with a spatula or large spoon. We want to be careful here to not mix too much to maintain as much of the fluffiness as possible.
Add a layer of strawberry sauce to four small containers. Then, divide the mousse evenly across them.
Chill them by either freezing the containers of mousse for 1 hour or refrigerating them for 2 hours. Top them with strawberry slices and enjoy them right away or save them for later in the refrigerator.