Preheat the oven to 180°C.
Boil water and granulated sugar in a saucepan until the sugar melts.
Add the syrup and 30g butter, bring back to the boil, and stir constantly.
Reduce heat, add all apples, and cook until just soft.
Transfer the apple mixture into an ovenproof dish.
Rub flour, 100g butter, and demerara sugar together until the mixture looks like breadcrumbs.
Spoon the crumble over the apples and bake for 15 minutes until golden-brown.
Whisk egg yolks and caster sugar together in a bowl.
Boil milk with vanilla seeds and pour into the egg mixture.
Whisk well then heat the custard slowly until it thickens enough to coat a spoon.
Serve portions of crumble with a dollop of warm custard.