Prepare the oven and dish. Preheat the oven to 375°F (190°C). Prepare a 9 × 13-inch baking dish.

Make the filling. Combine the mixed berries, sugar, cornstarch, lemon juice, vanilla, and optional salt. Spread the mixture evenly in the baking dish.

Mix the dry ingredients. In a bowl, whisk together the flour, sugar, baking powder, and salt.

Cut in the butter. Add the cold butter and work it into the flour mixture until pea-sized crumbs form.

Add the buttermilk. Pour in the buttermilk and mix gently until a soft dough forms. Avoid overmixing.

Assemble the cobbler. Drop spoonfuls of biscuit dough over the berries, leaving small gaps. Brush with the optional buttermilk and sprinkle with coarse sugar.

Bake. Bake for 40–45 minutes, checking at 40 minutes. The topping should be golden brown and cooked through, and the berry juices should be bubbling. Continue baking in 5-minute increments if needed.

Rest and serve. Let the cobbler rest for 10–15 minutes before serving so the filling can thicken.