Prepare the custard: In a large bowl, mix together the vanilla sugar and the 2 tablespoons of butter until the mixture combined. Add the eggs and whisk until they are thoroughly incorporated. Whisk in the milk, then sift the flour and the baking powder over the mixture, and whisk it in until the mixture is smooth and pale. Fold in the plums and any juices they’ve given off (unless there are cups and cups!), and pour the mixture into the prepared mold. If using individual molds, evenly divide the mixture among them - it should come about halfway up the side of the molds. Bake in the center of the oven until it begins to puff and look golden, 30 minutes for a large mold, 15 for individual molds.