I like to mix whipped cream mixed with some coconut milk, for the flavor and health benefits (more about the latter below). You can also add whiskey in the chocolate bake mix, if you want, and then forgo the coconut milk in the whipped cream.
If you know how to make brownies, then lose the flour and you have mousse. Note that you don’t have to use all the butter most recipes out there recommend. I use 30 to 40% less, and also use yogurt butter, which has 20% less fat than regular butter (at least mine does). What you do then for brownies is add a few tablespoons of water to the melting chocolate and butter mix. If you make this mousse, however, you don’t need to do anything else: just reduce the amount of butter if you follow other recipes, or follow the amounts I give below.