In a large bowl, toss blackberries with sugar, lemon juice and zest, cornstarch, vanilla, and salt. Let sit 30 minutes. Preheat oven to 350º. In another large bowl, whisk together flour, sugar, baking powder, and salt. Work in the butter with your hands until combined, then stir in milk and egg until just combined. Transfer the blackberry mixture to an oven-safe skillet and top with dollops of dough. Brush dough with egg wash and sprinkle with coarse sugar. Bake until biscuit topping is deeply golden and blackberry mixture bubbly, 45 minutes. Let cool 10 minutes before serving with vanilla ice cream.