Preheat the oven to 160C/gas 3/fan 140C and grease a 850ml/1½ pint shallow baking dish with a little butter. Cut the panettone into wedges, leaving the crusts on. Butter the slices lightly with the rest of the butter. Cut the slices in half and arrange them in the dish, buttered side up.
In a bowl, whisk together the eggs, cream, milk, vanilla extract and sugar and pour evenly over the panettone.
Put the dish in a roasting tin and pour hot water around it to a depth of about 2.5cm/1in. Bake for 35 minutes until the pudding is just set - it should be yellow inside and nicely browned on top. Dust with icing sugar and serve with spoonfuls of whipped cream.