1. Preheat the oven to 400ºF (200ºC). Generously butter a 6 to 8 cup (1,75l) baking or gratin dish, or a large pie plate. (I like to use one that’s narrower, but deeper.) Strew the plums over the bottom of the baking vessel.
2. In a medium bowl, whisk together the eggs, flour, 1/4 cup sugar, heavy cream, salt, and about a third of the milk, until there are no lumps in the mixture. Whisk in the rest of the milk, the kirsch, and vanilla extract.
3. Pour the custard over the plums in the baking dish. Bake on the middle rack of the oven until the custard is just barely set in the center, about 20 minutes.
4. Remove from the oven and set on a cooling rack. Wait a minute, then sprinkle 1 teaspoon of sugar over the top.