Heat the double/whipping cream until it bubbles around the edges but not boils yet. Pour over chopped chocolate, cover with a lid for a minute, then uncover and stir with a whisk until smooth, add mascarpone, chocolate hazelnut spread and orange liqueur, stir until well combined.
Pour into the chilled tart and put in the fridge to set for at least one hour.
When the tart is set, whip the double/whipping cream until soft peaks form, then add mascarpone and chocolate hazelnut spread and whip for another minute until it's pipeable consistency.
Load a piping bag fitted with a star tip and pipe rosettes around the edge of the tart or spread it and swirl with a spatula. Sprinkle with chopped hazelnuts.
Keep in the fridge but remove at least 30-40 minutes before serving to allow the chocolate come to a room temperature for maximum flavour.