To make the caramel, put the sugar and hot water into a small, heavy pan over a low-medium heat. Stir occasionally until the sugar has dissolved then raise the heat and boil, without stirring, until the caramel is golden brown.
Immediately remove the pan from the heat and quickly pour the caramel into a wide, fairly deep, 1.5 litre/2¾ pint ovenproof dish, swirling it around to cover the base. Set aside.
Preheat the oven to 160C/Gas 3/fan oven 140C. Cut the panettone into 1cm/½in thick slices then remove the dark brown, bottom crust. Butter one side of the slices generously. Put in a single layer, butter-side-up, in the dish.
Using an electric whisk or food mixer, beat the sugar and eggs in a large bowl for 2 minutes, or until thickened. Heat the milk and cream in a saucepan to just below simmering point, then pour on to the egg mixture and stir to combine.
Pour this custard over the panettone, pressing the slices into the liquid. Soak for 10 minutes.
Put the dish on to the middle shelf of the preheated oven, and bake for 55-60 minutes. The pudding is ready when the top is pale gold and dry-looking and the custard set to a just-firm shiver – it should not be completely solid as the custard continues to set as it cools. Leave it to rest for 30 minutes before serving with cream.