Hi Cynthia!

It's pretty tough to know exactly what is contributing to you fallen mousse problem. What I can tell you is that the secret to Chocolate Mousse - like so many great simple dishes - is to get just a couple of things exactly right. In this case, you need to be sure that you have whipped your eggs whites into a real meringue that hold stiff peaks. If it smooths out on top and looks like soft whipped cream, you won't have the hold you need to keep the mousse "aloft" as it sets. The whipped cream will help keep everything light and fluffy. Lastly, be sure not to beat the mousse to death when you are folding it all together. Just barely enough to combine and then let it sit. You will be in Chocolate Heaven!

David