Preheat the oven to 400̊F (205̊C). Thoroughly butter and flour a 2-quart (2L ) round baking dish about 4 inches (10 cm) deep, or 10 individual 1 cup (250ml) ramekins or molds.

Melt the 3 tablespoons of butter in a medium-sized skillet over medium heat. Add the plums and sauté just until they are hot through, about 4 minutes. Remove from the heat and add the Calvados, swirling it in the pan. If the Calvados doesn’t flame up, light it with a match, being sure to stand well back from the pan and averting your face. Reserve the plums.

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.

While the custard is baking, prepare the topping: Mix together the vanilla sugar and the butter until thoroughly combined and smooth, then whisk in the egg until it is thoroughly combined. Remove the baked custard from the oven and spread the topping over it. If using individual molds, spread an equal amount of topping over each one, and return to the oven to bake until it is golden and bubbling, an additional 20 minutes for either size mold. Remove from the oven and let cool to room temperature before serving.