Toss berries with sugar, lemon juice, zest, cinnamon, cornstarch in baking dish: Put the blackberries, sugar, lemon zest and juice, cinnamon and cornstarch in a 9x9 baking dish. Stir to combine everything and make sure that the berries are all evenly coated with the sugar.
Let sit for 30 minutes for the berries to macerate so that the sugar dissolves and the berries release their juices.
Preheat your oven to 350°F.
Make biscuit dough topping: Vigorously whisk together the flour, 3 tablespoons of the sugar, the baking powder and salt in a medium bowl. Cut the butter into the flour mixture with your fingers or a fork until the topping mixture resembles coarse crumbs.
Make a well in the center and stir in the milk and beaten egg until the dough is just moistened.
Drop spoonfuls of biscuit dough on top of berries: Scoop up the dough in large spoonfuls, and drop over the berries in the baking dish like cobblestones.
Bake: Bake for 30 minutes at 350°F, or until the berry mixture is bubbly and the topping is nicely browned.
Great served with whipped cream or vanilla ice cream.