Remove from the double boiler, add the clementine zest, salt and softened butter. Continue whisking at full speed for about 5 minutes. Make sure the eggs are whisked for a long time, so the mousse is light and airy.
Strain the liquid through a fine mesh sieve into a bowl to remove any solids (zest and pulp). Let cool to room temperature, cover with plastic wrap and refrigerate for at least 30 minutes.
Whip the cream (at low speed) for 2-3 minutes until creamy and smooth. Add the rest of the powdered sugar. Increase to maximum speed and keep beating for another 2-3 minutes, until it forms soft peaks. Do not over-beat or the texture will become grainy.
Pour 1/3 of the whipped cream into the clementine custard. Mix well using a silicone spatula. Pour in the rest of the whipped cream; gently fold the cream into the custard to get an airy mousse.