Preheat the oven to 180°C.
Butter a 1.5 litre ovenproof dish with half of the butter.
Spread the bread slices with the remaining butter and cut into triangles.
Layer the bread and sultanas into the buttered dish.
Whisk egg yolks, whole eggs, and caster sugar in a bowl until creamy.
Mix in 300ml of double cream, milk, vanilla, and Irish cream liqueur.
Pour the custard over the bread and press down until submerged.
Leave to soak for 20 minutes.
Place the dish in a roasting tray filled halfway with water.
Sprinkle with demerara sugar and bake for 35-40 minutes.
Serve warm with extra double cream poured over.