Preheat the oven to 180°C and grease an ovenproof dish with butter.
Rub the butter and flour together in a bowl until the mixture resembles breadcrumbs.
Mix in the demerara sugar and set the crumble topping aside.
Simmer whole pears in water with 50g of sugar for 12 minutes until tender.
Drain the pears, allow them to cool, then remove cores and chop the flesh.
Layer the chopped pears, blackberries, and remaining caster sugar into the dish.
Top with crumble and bake for 25 minutes until golden-brown and bubbling.
Whisk egg yolks and sugar in a bowl until thick and pale.
Boil milk and cream, pour over eggs while whisking, then heat in a pan until thickened.
Divide the crumble into bowls and serve topped with warm custard.