To make the custard, mix the milk, butter, 50g caster sugar, the lemon zest and vanilla extract in a saucepan. Gently heat until the sugar has dissolved and the milk is steaming but not boiling. Remove from the heat and leave to cool.
In a mixing bowl, beat the egg yolks, then slowly whisk in the cooled milk mixture. Stir in the breadcrumbs. Pour the mixture into the greased pie dish, then leave to soak for 20 minutes.
Heat the oven to 150°C/130°C fan/gas 2.
Bake the custard for 35 minutes or until lightly golden and set, then remove from the oven.
When the custard is ready, make the meringue. Put the 2 reserved egg whites into a clean mixing bowl, add a pinch of salt and use an electric hand mixer to whisk the whites to stiff peaks. Whisk in the 100g caster sugar, a tablespoon at a time, to form a thick, glossy meringue.
Heat the oven to 150°C/130°C fan/gas 2. Spread the jam over the baked custard, then top with the meringue. Bake for 20-25 minutes until crisp on top. Serve hot with cream, custard or ice cream.