Preheat the oven to 180C/350F/Gas 4. Lightly oil a 900 ml ovenprof dish. Peel, core and slice the apples and place in a suacepan with the 50 g caster sugar, lemon rind and 2 tablespoons water. Simmer for 10-15 minutes or until tender.
Mix the breadcrumbs with the demerara sugar and the cinnamon. Place half the sweetened apples in the base of the prepared dish and spoon over half of the crumb mixture. Place the remaining apples on top and cover with the rest of the crumb mixture.
Melt the butter and pour over the surface of the pudding. Cover the dish with non-stick baking paper and bake in the preheated oven for 20 minutes. Remove the paper and bake for a further 10-15 minutes or until golden.
Meanwhile, make the custard by whisking the egg yolks and 1 tbsp caster sugar together until creamy. Mix 1 tablespoon of the milk with the cornflour, until a paste forms and reserve.
Warm the rest of the milk until nearly boiling and pour over the egg mixture with the paste and vanilla essence.
Place the bowl over a saucepan of gently simmering water. Stir over the heat until thickened and can coat the back of a spoon. Strain into a jug and serve hot over the pudding.