Whisk the yolk and sugar until light and fluffy and doubled in size, about 3 mins. Beat in the Cointreau, lemon and the flesh of 2 passion fruits. Softly whip the cream until it just holds some shape (it stiffens quickly towards the end so whip slowly as it gets thicker). Chill 2 dessert glasses.
To assemble, gently fold the egg mix into the double cream, having first given the eggs another brisk whisk. Spoon the cream into little glasses and top them with the seeds from the remaining passion fruit and a drizzle of Cointreau.