Preheat the oven to 400 degrees. Arrange 2 racks equally spaced in the oven. Place six (6 1/2-inch) individual gratin dishes on 2 sheet pans.
In a small bowl, combine the vanilla seeds with the 2 teaspoons of sugar. Butter the six gratin dishes well with the butter and sprinkle the dishes with the vanilla and sugar mixture. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat the eggs and the 2/3 cup sugar on medium speed, until blended. Lower the speed and add the flour, cream, milk, and Armagnac. Whisk in the star anise and salt just until combined and set aside for 10 minutes. The batter will look like pancake batter.
Cut the plums in half through the stem, discard the pits, and cut each half in wedges, again cutting through the stem. Distribute half a plum to each gratin dish, placing them in a pinwheel pattern. Distribute the batter evenly in the gratins (the plums will only be half immersed in the batter) and bake for 35 to 40 minutes, until the edges are golden brown and the custard is just set.
Sprinkle with the orange zest and serve hot or warm.