1.
Slice the croissants thickly, then spread with the butter. Arrange them in a 1.7 litre shallow baking dish. Scatter in the sultanas.
2.
To make the custard, pour the milk and cream into a saucepan with the vanilla, and heat over a very low heat until warm and well-flavoured.
3.
Meanwhile, in a large bowl, whisk the egg yolks with the sugar until light and foamy. Strain the warm milk onto the egg mixture, whisking all the time.
4.
Pour the egg mixture evenly over the croissants. Place the dish in a bain-marie, or larger roasting pan, and pour in enough boiling water to come half-way up the sides of the dish. Bake for 45-50 minutes until the custard is set and the top is golden.
5.
Leave the pudding in the bain-marie until it's cooled a bit, then enjoy.