Heat the oven to 350°F. Butter a cast iron skillet and set aside. Trim the plums into wedges that are 1/4 inch wide on the skin side of the cut. Toss the plums with 1/3 cup of the granulated sugar, and arrange them evenly in the skillet and set aside. In a medium bowl, whisk the eggs with the remaining 1/3 cup sugar, flour, and salt. Whisk in the milk and vanilla. Pour the batter into the skillet, and place on the center rack in the oven. Bake just until set, about 1 hour. When cooled, remove from the skillet and slice into wedges. Dust with confectioners' sugar before serving.