1. Preheat oven to 400. In a medium bowl, mix the plums, 1/4 c of sugar, cinnamon and vanilla until combined. Spoon into a 1 1/2 - 2 quart ceramic or glass baking dish. Set aside.
2. In another bowl, whisk together the flour, baking powder, baking soda, salt and 1/2 c sugar. Add the buttermilk and melted butter, and mix until just combined, being careful not to overmix.
3. Using a large spoon, dollop the batter onto the plums. (Don't worry about covering the entire surface - the batter will spread when it bakes.)
4. Bake for 25 to 35 minutes or until the cobbler topping is golden and set. Remove from the oven, and immediately sprinkle the top with 1 teaspoons sugar. Serve warm, topped with a scoop of vanilla ice cream, if desired.