Beat pudding mix and cold milk in mixing bowl on medium-low (or whisk) for 1 to 2 minutes. Beat in the berries (if using frozen, slightly thaw so you can break them up) and whipped topping on lowest speed.
Spoon mousse into 4 individual custard cups and top each with 1/3 cup of cereal.
Serve immediately, or cover and keep refrigerated until ready to serve (ideally, they should be served within a couple of hours).
