- Preheat the oven to 375 degrees. Butter a 10-inch cast iron pan, and sprinkle 1 tablespoon of sugar over the bottom.
- Arrange the plum halves, cut side down, so that they cover the entire bottom of the cast iron. Sprinkle 2 tablespoons of sugar over the top of the plums. In a large bowl, blend the remaining 3 tablespoons of sugar, eggs, milk, flour, lemon zest, vanilla, salt, and cinnamon. Pour over the fruit in the pan.
- Bake for 50 to 60 minutes, or until firm and lightly browned. Let stand 5 minutes before slicing. Dust with confectioners’ sugar before serving.