In a small bowl, combine the plums and sugar and let sit at room temperature for at least 30 minutes.
Preheat the oven to 350 degrees F. Butter an 8-inch cast iron skillet or an 8 by 8-inch square baking pan. In a large mixing bowl, combine all of the dry ingredients.
In another mixing bowl, combine all the wet ingredients and whisk to thoroughly combine. Make a well in the dry ingredients and pour the wet ingredients into the well. Mix gently to combine. When the mixture has just come together, transfer it to the prepared skillet or baking pan. Sprinkle the plums in an even layer over the top of the batter. Bake in the preheated oven for 25 to 30 minutes. Remove from the oven and let cool before cutting. Serve topped with vanilla ice cream.