Whip the heavy whipping cream until it firmly holds peaks. Set aside in the refrigerator.
If your eggs aren't pasteurized, heat eggs and sugar in a double boiler until they reach 140 F, stirring the whole time. This will kill any salmonella bacteria. If you are using pasteurized eggs, skip this step.
Whisk the eggs and sugar until very fluffy and thick - about 10 minutes on high speed in a stand mixer.
Mix the butter into the melted chocolate until it is all melted.
Add the chocolate/butter mixture a little bit at a time into the egg/sugar mixture, stirring well to fully incorporate the chocolate. I do not suggest using an electric mixer as it is very easy to overmix using that method.
Fold in the whipped cream.
Pipe onto desserts. Refrigerate until ready to serve.
If you want to get fancy like I did, you can drizzle some melted chocolate on top of the mousse frosting. You won't regret that choice!I adapted the mousse recipe from Ghirardelli's Chocolate Mousse Recipe.