Preheat oven to 375°F; butter a 9x13-inch dish.
Toss plums with 1/2 cup sugar, cornstarch, and vanilla; spread evenly in dish and stand 10 minutes.
Whisk flour, remaining 1/4 cup sugar, baking powder, and salt in a large bowl; cut in butter with pastry cutter or fork to coarse crumbs.
Stir in milk just until combined; drop dough spoonfuls over plums.
Sprinkle with coarse sugar; bake 40-45 minutes until topping is golden and fruit bubbles.
Cool at least 15 minutes; serve warm with ice cream or whipped cream.