Preheat the oven to 375°F (190°C). In a mixing bowl, combine the mixed berries, sugar, cornstarch, vanilla extract, and lemon juice. Toss until the fruit is well coated.

Pour the fruit mixture into a greased baking dish.

In another bowl, whisk together the flour, baking powder, sugar, and salt. Cut in the cold butter until the mixture resembles coarse crumbs.

Stir in the milk until just combined. Do not overmix.

Drop spoonfuls of the biscuit topping over the fruit filling, spreading it out evenly.

Bake in the preheated oven for 30-35 minutes, or until the topping is golden brown and the fruit is bubbly.

Let cool slightly before serving. Enjoy warm with ice cream if desired.