Preheat your oven to 350°F and butter a 9x13-inch baking dish.

Combine all the berries with 3/4 cup sugar, cornstarch, and lemon juice in a large bowl, then transfer to the prepared baking dish.

In a separate bowl, whisk together flour, remaining 1/4 cup sugar, baking powder, and salt, then cut in the cold butter until the mixture is crumbly.

In a small bowl, whisk together buttermilk, egg, and vanilla extract, then pour into the flour mixture and stir until just combined.

Drop large spoonfuls of the batter over the berry mixture, leaving some gaps for steam to escape.

Bake for 45-50 minutes until the topping is golden brown and the berry mixture is bubbling vigorously.

Let cool for at least 20 minutes before serving to allow the filling to thicken properly.