Soak the gelatine in some cold water to soften. Set aside.
Meanwhile, heat the milk together with the Chios Mastiha powder and remove from heat as soon as the milk begins to boil. Strain the gelatine into the milk, stirring constantly. Cool slightly.
With an electric whisk, beat the egg yolks and sugar together at high speed until they are creamy and frothy. Pour the milk very slowly into the egg mixture, whilst beating at high speed. Set the bowl over a larger bowl filled with ice, and continue beating until the mixture gets very frothy.
Whip the cream until stiff peaks form and fold it carefully into the egg-milk mixture. Pour the mixture into a bowl or into a loaf tin lined with cling film.