Leave the tartlet shells on the counter to thaw while you quickly whisk together the mousse.
In a large bowl combine the pudding mix and milk. Whisk until smooth and combined. Add the cool whip and raspberries and gently fold them into the pudding mix to make it light and fluffy.
You can spoon or pipe the mousse into the tartlet shells. Keep refrigerated until ready to serve.
Notes
The mousse can be made up to 3 days ahead of time, but I wouldn't put it into the tartlet shells more than a day in advance.