Put cream and mint in a small saucepan and bring to the boil; remove pan from heat and let mixture steep for 20 minutes. Meanwhile, chop chocolate into small pieces. Strain cream back into saucepan, discarding mint leaves. Reheat slowly, gradually stirring in chopped chocolate until melted and mixture is smooth. Remove from heat and set aside.
Beat egg whites until stiff peaks form. Set aside.
In a separate bowl beat egg yolks and sugar until mixture is frothy and light coloured.
Pour chocolate mixture into egg yolk mixture and mix well, then fold in 1/3 of whipped egg whites and mix. Finally, gently fold in remaining egg whites, stirring very carefully till no white streaks remain.
Cover with cling film and chill in fridge for at least 12 hours.