Preheat the oven to 170°C (340°F) and butter a medium-sized ovenproof dish.
Slice the hot cross buns into slices horizontally and butter each cut side.
Arrange the buns in the dish, overlapping slightly. You can stand them up or lay them flat — rustic is beautiful!
Whisk together the eggs, milk, cream, sugar, and vanilla in a jug or bowl.
Pour the custard over the buns slowly, making sure each piece soaks it up. You want the bun slices to be covered in the custard.
Get a roasting pan that is bigger than your dish with the pudding in it. Pop the pudding dish into the roasting pan and place it into the oven. Add hot water to the roasting pan and add enough that it goes halfway up the side of the pudding dish. This is an important step; the water surrounding the pudding dish makes for a nicely cooked custard.
Bake for 30–40 minutes, or until the custard is just set and the tops of the buns are golden brown.
Cool slightly. Then, dust with icing sugar. Serve warm with a scoop of vanilla ice cream. Alternatively, you can drizzle cream on top.