Preheat oven to 375°F (190°C). Set aside a 1 & 1/2- to 2-quart baking dish.
Combine the plums, granulated sugar, cornstarch, and cinnamon. Mix directly in the baking dish, or mix in a separate bowl and transfer to the baking dish.
Whisk together the flour, brown sugar, baking powder, and salt.
Add the butter to the flour mixture, and mix with a pastry blender, a fork, or your fingers until crumbly.
Add the milk, and mix just until a dough forms.
Using about a tablespoonful at a time, drop the dough over the top of the plum mixture. If you like, sprinkle the top with coarse sugar.
Bake 35 to 40 minutes, or until the topping is golden brown and the filling is bubbly.
Notes
Store any leftovers tightly sealed in the refrigerator up to 3 days.