Chocolate – Where to get or how to make hollow sugar balls

The dessert pictured above definitely, definitely has gelatin in it. That will change the mouth feel on the mousse slightly, but it might be what you are going for and will provide some of the stability that you are looking for. To have it hold a form like this you will need a recipe that requires gelatin; I would recommend gelatin sheets if you can find them at a local baking/cooking supply store or order them online.

The fluffiness will also be determined by several things. How were the whites whipped (too much, too little) or how was the cream whipped (again too much, too little)? There are recipes that call for both. How you incorporate all of the ingredients into one another really matters and in what order, this is called tempering. You want to fold until the mixtures are just combined; no more.

Temperature is of key importance for all of the ingredients - some you want very cold while others (ie chocolate) you want to be room temperature but not too cool or it will seize up the mixture and the mousse will taste grainy.

Also chocolate mousse texture can definitely be affected by the percentage and quality of chocolate you are using. If is is of a higher percentage it can create a denser mousse and deflate the whites and cream more easily.

A simple alternative would be to make a ganache flavored with Frangelico or another hazelnut liqueur. This has the advantage of having much less sugar, and probably better overall flavor, than an even-more-sweetened Nutella, assuming you start with a reasonably dark chocolate. All you do is boil cream, and pour it over a similar quantity by weight of chopped chocolate, into which you've added a healthy splash of hazelnut liqueur (or the liqueur can go into the cream as you bring it to a boil; either way works in my experience). Then stir vigorously until all the chocolate is smoothed out. You can pour it over while the mixture is still moderately warm and refrigerate it with the cheesecake. It will not be Nutella, but from experience it will be good, and evoke the same basic flavors.

Alternatively, most cheesecakes I am familiar with aren't topped with an icing, but a mixture of say cream or sour cream and other ingredients, sometimes added in the last few minutes of baking. These usually spread out reasonably well. You could consider softening the nutella in the microwave and mixing with a bit of cream, which will be a sort of less heavy ganache.