Place Nutella in a large bowl. Microwave it for a few seconds to soften it, then let it cool slightly.
In a mixer bowl fitted with the whisk attachment, whisk the heavy cream until soft peaks form. Add 3 tablespoons of the whipped cream to the Nutella and whisk until smooth. Gently fold in the rest of the whipped cream until combined and smooth. Transfer to a trifle bowl, or divide evenly between serving cups.
Chill the mousse at least 2 hours until set. Garnish with chocolate chips/shavings, hazelnuts/nuts or Ferrero Rocher. Store in the fridge, covered, for up to 4 days.