Blackberries
In a large bowl, toss blackberries, granulated sugar, lemon zest, lemon juice, cornstarch, vanilla, and salt. Let sit 30 minutes.
Cobbler biscuits
Arrange a rack in center of oven; preheat to 350º. In another large bowl, whisk flour, granulated sugar, baking powder, and salt. Work in butter with your fingertips until combined, then stir in milk and 1 egg until just combined.
Transfer blackberry filling to 10" ovenproof skillet (preferably cast-iron) and top with dollops of dough. In a small bowl, beat 1 egg with a splash of water. Brush dough with egg wash and sprinkle with coarse sugar.
Bake cobbler until biscuit topping is deeply golden and blackberry mixture bubbly, about 45 minutes. Let cool 10 minutes. Serve with vanilla ice cream alongside.
