Wet the base and sides of a loaf pan. Cover sides and base with plastic wrap (water helps wrap cling to the pan).
Melt chocolate in a double boiler or in the microwave. Set aside to cool slightly.
In the bowl of a stand mixer, beat egg, egg yolks and caster sugar until creamy and triple in volume. Transfer mixer bowl on top of a pot with a little simmering water. Don’t let the bowl touch the water. Stir constantly with a whisk until the mixture reaches 65 C.
Take the bowl off the heat and add the espresso and vanilla. Fold with your whisk until combined. Add the melted chocolate and mix gently until combined.
Whip cream to soft peaks. Add cream to the mixture and fold in until combined.
Transfer about 1/2 of the mixture in the loaf pan. Swirl in 1/2 of the Nutella. Top with the remaining mixture and swirl in the remaining Nutella.
Cover the loaf pan with the overhanging plastic wrap. Freeze for at least 8 hours.
To serve, revert on a platter, pour over some chocolate ganache and sprinkle with the “pastellaki”.