Arrange half the plum slices in the base of a 1.5 ltr (2½pt) baking dish. Use 1 tsp butter to grease around the sides and rim of the dish. Spread the remaining butter over the slices of panettone or brioche. Cut into triangles, then arrange in the dish, overlapping to fit. Tuck the remaining plum slices in among the bread.
Beat together the eggs, milk, single cream, caster sugar and vanilla extract. Allow a few minutes for the sugar to dissolve. Pour slowly into the dish, over the bread. Cover and allow to stand for at least 30 minutes. (If you want to cook the pudding later, refrigerate until required).
Preheat the oven to 180°C/Gas Mark 4. Stand the baking dish on a baking sheet.
Bake for 30-35 minutes until puffed up and golden brown. Cool for a few minutes, then serve with whipped cream or mascarpone.