Preheat oven to 375 degrees F. Combine berries, sugar, cornstarch, salt and let sit to macerate for 20 minutes while oven preheats, tossing a few times with a rubber spatula. Dump into a large cast-iron skillet, pie dish, or baking dish.
Bake the mixed berry filling for 25 minutes, until bubbling around the edges. While the filling bakes, prepare the biscuit topping.
In a large bowl, combine the flour, sugar, baking powder, baking soda, salt, and cinnamon and whisk together. In a separate bowl, combine the butter, buttermilk, and vanilla extract and whisk together. Combine the cinnamon and sugar in a small bowl.
Just before the filling is done baking on its own, gently pour the buttermilk mixture over the flour mixture and mix them together with a fork just until moistened. Then turn onto a clean surface and knead once or twice, and divide into 12 equally sized lumps. Gently shape or pat into balls with your hands.
Remove the pan from the oven with the berry filling and set the balls of biscuit dough on top of the berries, then brush the dough with a little milk and sprinkle with the cinnamon sugar mixture. Return the pan to the oven and increase the temperature to 425 degrees F. Bake for 15-18 minutes, then remove from oven and let cool for at least 1 hour before serving.