First make the meringues:
Preheat a very cool oven, Gas Mark ½, 125°C (250°F). Butter and flour a large baking sheet. Whisk the egg whites until stiff, add the caster sugar slowly, whisking until completely dissolved, then lightly fold in the icing sugar. Pipe the mixture into small circular shapes and bake in the low oven for about 1½ hours to dry out without colouring. Cool and store in an airtight container.
Next make ice cream:
Heat the milk with the granulated sugar and split vanilla pod (scrape seeds into the milk). When just boiling, remove from the heat, cover and leave 10 minutes to infuse. Meanwhile beat the egg yolks with the caster sugar until the sugar has completely dissolved and the mixture is creamy. Bring the milk infusion back to the boil, add to the yolk mixture, keep whisking and return to the rinsed saucepan. Cook over medium heat, whisking all the time, to thicken a little – be very careful not to overcook – then strain through a very fine sieve. Whisk softened gelatine into the mixture and allow to cool, whisking occasionally. When cold, churn the mixture in an ice cream maker until thick.
To marinate the blackberries:
Sort through the blackberries, removing any stems; wash and drain. Put sugar and water into a pan and heat to dissolve, then bring up to the boil. Allow to cool, then add the blackberries.
To assemble the terrine:
Line two small loaf tins, 18cm x 6.5cm (7″ x 2½”) with overlapping clingfilm. Break up the meringues and fill the bottom of each tin, add ice cream, then some of the marinated blackberries. Cover with another layer of ice cream, close with overlapping clingfilm and freeze.