Chocolate: The chocolate you choose will determine the sweetness of the chocolate mousse. Use semi-sweet for a sweeter treat or increase the darkness of the chocolate for a more refined cacao-rich taste.
Hazelnuts Option: If you are unable to find hazelnut butter, you can process hazelnuts in a spice grinder or food processor. They will first become finely ground, and then will release their oils to become a paste / butter.
Thicker Mousse: I like the softer version above, which also freezes best, but if you like a denser chocolate mousse, reduce the hazelnut creamer to ⅔ cup and reduce the hazelnut butter to 2 tablespoons.