Preheat the oven to 180°C/Fan 160°C/Gas 4. Grease a 1 Litre oven-proof baking dish with butter and set aside.
Pop the hot cross buns onto a board and spread the bases generously with raspberry jam. Sprinkle over half the fresh raspberries, followed by half the chopped white chocolate. Top with the lids to make 8 hot cross bun sandwiches.
Arrange the hot cross bun sandwiches into the prepared dish - you may have to cut some into quarters so that they all fit neatly (try to fill in any gaps). Scatter over the remaining raspberries and white chocolate. Set aside.
In a large bowl, whisk together the eggs and sugar until the sugar has dissolved. Set aside.
Pour the milk and cream into a large saucepan and place over a low heat. Warm through until beginning to steam. Take care not to let it boil. Remove from the heat and gradually pour over the egg-sugar mixture, whisking constantly.
Carefully pour the egg mixture over the hot cross buns, ensuring they are evenly covered. Leave to stand for 10 minutes to allow the hot cross buns time to soak up some of the custard.
Place the baking dish into a deep roasting tin and pour in enough cold water round the edge to come about 2.5 cm up the sides of the dish (take care not to get any water into the pudding itself).
Pop in the oven and bake for about 30 minutes - or until the custard is just set and the top is golden and crisp.
Leave to stand for 10 minutes before spooning into bowls and serving warm with scoops of vanilla ice cream or a drizzle of double cream. You can also add the ice cream on top of the dish and serve family-style at the table.