For the mousse:
For the candied mandarin:
Whisk eggs and sugar together in a heatproof bowl set over a pot of simmering water. The bowl should not touch the water. Whisk until the volume of the mixture doubles and is hot to the touch. Remove from heat.
Stir in chocolate, cocoa, mandarin zest, juice, salt and then the butter. Let the mixture cool to room temperature.
Whip the thickened cream to stiff peaks. Fold a third of the cream into the chocolate mixture to lighten it. Then fold in the remaining whipped cream.
Divide the mousse into 4 serving glasses. Refrigerate at least 1 hour and up to 1 day to set.
For the candied mandarin thinly slice the peel of 1 mandarin. Whisk together mandarin juice and sugar, pour into a small saucepan. Add the mandarin peel and boil until the peel is translucent (8-10 minutes). Leave to cool and store in the syrup in the fridge until ready to serve.
To serve top each glass of chocolate mousse with some lightly whipped cream and some candied mandarin peel and fresh mandarin segments.