3 tbsp ready-made elderflower cordial
4 passion fruits, seeds scooped out, skins discarded
600g/1lb 5oz rhubarb, cut into small chunks
50g/2oz caster sugar, plus extra to sweeten if necessary
150ml/5fl oz ready-made custard
150ml/5fl oz double cream
2 free-range egg whites, whipped to form soft peaks