To make the mousse place the 2 egg yolks with the orange zest into the bowl of an electric mixer (keep the whites for later). Add the remaining 2 whole eggs and the caster sugar and whisk on high ...

Fold in the stiff egg whites until they too are combined. Spoon the mousse into 8 small jars or ramekin dishes and place into the fridge to firm up. Top with some whipped cream just before serving.