Preheat oven to 350°F. Butter baking dish and set aside.
Remove pits from plums and cut into thin slices. Place in a medium bowl with 1/4 cup sugar and vanilla bean husk and toss. Set aside, allowing the fruit to macerate at least 10 minutes.
In another medium bowl, whisk sugar, flour, and salt. Add eggs, milk, vanilla bean seeds and whisk until a smooth batter forms.
Arrange macerated plums in the bottom of your baking dish. (You can add the vanilla bean husk if you want, but keep in mind you will have to remove it after it bakes as it is inedible.) Pour batter into the skillet and place on the center rack in the preheated oven. Bake until set, between an hour and an hour and 10 minutes, or until lightly golden brown and puffy.
Allow to cool before slicing into wedges. Dust with powdered sugar and/or add a dollop of ice cream or vanilla yogurt and serve immediately. May be frozen up to one week, and keeps well up to four days in the fridge.