For the lemon curd:
Put lemon zest and juice in heatproof bowl. Add butter and remaining caster sugar, stir well, and set over pan of simmering water. Whisk until totally dissolved then beat in whole eggs. Whisk until thickened and cooked through. Remove from heat and cool over bowl of iced water, whisking occasionally until completely cold. Whip the double cream in large bowl until soft peaks form. Gently fold in lemon curd. Scoop out seeds of 6 of the passion fruit and fold lightly into cream.
To assemble: place meringue disc on serving plate, then top with half the lemon curd cream. Place second disc on top and spread over rest of cream. Place last disc on top and press very lightly to settle. Scoop out seeds from remaining passion fruit and spoon over top. Dust with a little icing sugar and cut into wedges to serve.