Preheat your oven to 375°F (190°C). Grease an 8x8 inch baking dish with a bit of butter or olive oil.
In the greased baking dish, combine the halved damsons, sugar, cornstarch, and vanilla. Stir gently so the fruit gets evenly coated, then place the dish in the oven for about 5 minutes until the fruit is hot and bubbling slightly around the edges. This step gives you that crucial steam layer — don't skip it.
While the fruit heats up, make the topping: whisk together the flour, sugar, baking powder, and salt in a mixing bowl. Cut in the chilled butter pieces using your fingers or a pastry blender until the mixture looks like coarse crumbs with some pea-sized lumps of butter still visible — this is what makes it tender rather than cakey.
Pour in the milk and stir just enough to bring the dough together; don't overmix. It should look shaggy and slightly sticky.
Remove the hot fruit from the oven. Drop spoonfuls of the topping over the bubbling fruit — you want about 12-15 mounds, leaving some gaps between them for steam to escape. Don't try to make perfect circles.
Bake for 30-35 minutes until the topping is golden brown and puffed, and a knife inserted into the center comes out clean. Let it rest for 5 minutes before serving so the fruit can set a bit.