I have a mousse recipe that uses a raw egg meringue. Can I use a cooked meringue instead? If I can, do I use the same amount? Will there be a difference?
No, you cannot use cooked meringue to make a mousse. Cooking meringue turns it from a sort-of liquid to a sort-of solid, and you would not be able to fold the other ingredients into a solid meringue. The meringue would just break up into little bits of foam that would be strewn about with the other ingredients, instead of producing the smooth, completely integrated foam that you are looking for. And it would look like a mess.
If you are concerned about consuming raw eggs, you can certainly use meringue powder, which is pasteurized, to make your mousse. The other option and you will have to figure out how to integrate it into the recipe you are using is to make the meringue with a sugar syrup at about 235°F (113°C), raising the temperature of the meringue to a safe temperature of around 160°F (71°C).
Egg White Safety
Substituting Egg Whites for Powdered Egg
What is Meringue Powder?
Substitute for Meringue Powder
How to Make Soft Meringue
How to Make Italian Meringue
How to Make a Safe Meringue
Egg White-Based Frostings