Preheat oven to 400 degrees F.
Place cleaned blackberries in a small bowl. Toss with 1/2 cup sugar, lemon juice, water, and cornstarch. Set aside.
Mix flour, baking powder, salt, and 1 tbsp sugar in a medium bowl. Cut in 2 tbsp butter with a pastry blender until coarse crumbs are formed.
Pour berries into a small (4 cup) baking dish. Dot with 1 tbsp butter.
Drop biscuit dough by spoonfuls over the berries.
Bake for about 20 minutes, until fruit is bubbling and biscuit topping is browned. Serve warm with vanilla ice cream.