Preheat the oven to 180°C, fan 160°C, gas mark 4. Grease 6 dariole moulds or ovenproof ramekins with the butter, then sprinkle a little of the caster sugar in each.
Put half the fruit in a pan with 20g of the remaining sugar. Bring to a simmer and cook for 5 minutes, then transfer to a food processor. Whiz until smooth then pour back into the pan.
Mix the cornflour with the lemon juice then stir into the fruit purée with the lemon zest. Cook for 2 minutes until thickened, then pour into a bowl and allow to cool slightly.
Beat the egg whites with a hand-held electric whisk until soft peaks form. Whisk in the remaining sugar until stiff peaks form.
Gently fold in the purée, then spoon the mixture into the moulds and smooth the tops with a knife.
Sit the moulds in a roasting tray, then pour enough boiling water into the tray to come half way up the sides of the moulds. Bake for 15 minutes.
Allow to cool for 1 minute, then turn out on to plates. Drizzle with the melted white chocolate and serve with the remaining berries.