Preheat the oven to 375 degrees F and butter a 2-quart baking dish. Make the filling: Toss the blackberries, sugar, maple syrup and vanilla in a medium bowl. If the berries are juicy, toss with the flour. Transfer to the prepared baking dish.
Make the topping: Whisk the flour, sugar, baking powder and salt in a large bowl. Using your fingers, work the butter into the flour mixture until it looks like fine meal; set aside.
Whisk the egg, buttermilk and vanilla in a small bowl; stir into the flour mixture until just incorporated. Drop heaping spoonfuls of the batter evenly over the berries and sprinkle with sugar.
Set the dish on a baking sheet and bake until the topping is golden brown and the filling is bubbling, about 40 minutes. Let cool slightly before serving.