Preheat oven to 350°F; butter a 9x13-inch dish.
Combine berries, 3/4 cup sugar, cornstarch, and lemon juice in a large bowl; transfer to dish.
Whisk flour, remaining 1/4 cup sugar, baking powder, and salt in another bowl; cut in cold butter until crumbly.
Whisk buttermilk, egg, and vanilla in a small bowl; stir into flour mixture just until combined.
Drop large batter spoonfuls over berries, leaving steam gaps.
Bake 45-50 minutes until topping is golden brown and berries bubble vigorously.
Cool at least 20 minutes to thicken filling before serving.