In a 8x8 pan or a pie dish or a cast iron skillet, combine the blackberries with sugar, lemon juice, vanilla or cinnamon, and cornstarch. Stir to combine, and set aside.
Preheat the oven to 350°F/180°C.
In a bowl, combine flour, baking powder, salt, and sugar. Then add the butter, and using a pastry cutter or a fork or your hands, cut into the butter so the mixture resembles coarse crumbs.
Add the beaten egg and milk, and stir with a spatula to combine.
Using a cookie scoop or 2 spoons, spoon the dough into portions over the fruit. Sprinkle with coarse sugar if using.
Bake for 30 minutes or until the fruit is bubbly, and the biscuit topping is golden brown (can be checked with a toothpick). Allow to cool for 5 minutes before serving, serve with vanilla ice cream or whipped cream.