1) Preheat the oven to 160C/Gas 3.
2) Combine the warmed milk and melted butter with the sugar, eggs and vanilla in a large mixing bowl. Whisk until incorporated and smooth.
3) Line a large baking dish with two-thirds of the challah slices. Sprinkle with half of the chocolate chips and half of the sultanas. Top with the remaining challah slices, layering them one on top of the other, and then the remaining chocolate chips and sultanas, making sure the chips and sultanas get inside the layers created by the challah slices.
4) Pour the milk mixture slowly over the top of everything and let the pudding stand until the bread has absorbed almost all of the liquid, 5 to 10 minutes.
5) Wrap the baking dish tightly with aluminium foil and bake for 50 minutes. Remove the aluminium foil and cook for 10 to 15 minutes longer until the pudding is set in the middle and lightly browned on top.
6) Remove from the oven and let it cool before serving. Serve with the chocolate ganache drizzled over the top.
For the chocolate ganache:
1) In a small saucepan, heat the cream to just below a simmer.
2) Place the chocolate chips in a medium heat-proof bowl and pour the hot cream over the top.
3) Cover tightly with cling film and let sit for 5 minutes.
4) Remove the cling film and whisk vigorously until the ganache becomes a uniform smooth chocolate sauce. Serve while warm.
Cook's note: To make the ganache in advance, let it cool, cover it tightly and keep it refrigerated until ready to use. Then microwave on low heat for 20 to 30 second intervals or heat over a double boiler.