Heat the oven to 375°F and butter a 3-quart baking dish.
Toss the plums with brown sugar, cornstarch, lemon juice, cinnamon and 1/4 teaspoon salt.
Spread the fruit in the dish and bake for 10 minutes while preparing the biscuits.
Whisk the flour, granulated sugar, baking powder and 1/2 teaspoon salt; cut in the butter and stir in the buttermilk.
Drop eight rough biscuits over the hot plums, leaving visible patches of fruit.
Bake for 30 to 33 minutes until the biscuits are browned and the plum juices bubble; cool for 25 minutes.