Adjust an oven rack to the middle position, and preheat the oven to 400℉.
PREPARE THE FRUIT:
In a large bowl, combine the plums with the cornstarch, sugar, vanilla and cinnamon. Toss well to coat. Place the plum mixture in a 9-inch deep-dish pie plate. Then place the pie plate on a foil-lined, rimmed baking sheet and bake until the fruit begins to release liquid (20 to 30 minutes).
PREPARE THE COBBLER TOPPING:
While the fruit is baking, in a large bowl whisk together the flour, ¼ cup of the sugar, baking powder, baking soda and salt. In a separate bowl, whisk together the buttermilk, melted butter and vanilla. In a third bowl, combine the cinnamon with the remaining 2 teaspoons of sugar.
Once the fruit filling has begun to release liquid, gently stir the buttermilk mixture into the flour mixture with a rubber spatula until the dough is just combined and no dry pockets remain.
Remove the pie plate from the oven, and stir the fruit. Pinch the cobbler biscuit dough into 8 equal pieces and place them on top of the hot filling, spaced ½-inch apart. Sprinkle the dough with the cinnamon sugar.
Return the cobbler to the oven and baking until the filling is bubbling and the biscuits are golden brown on top and cooked through (15 to 20 minutes)
Serve warm with a scoop of vanilla ice cream (if desired).