Preheat oven to 375°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.
6 cups mixed berries, ½ cup granulated sugar, 2 Tablespoons cornstarch, Pinch of salt
In a large bowl, combine the flour, sugar, baking powder, baking soda, salt, and ¼ teaspoon cinnamon and whisk together. Add the cubes of butter and mix to combine. Combine the remaining 2 Tablespoons sugar and ¼ teaspoon cinnamon in a small bowl and set aside.
2 cups (282g) all-purpose flour, ½ cup (100g) granulated sugar, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt, ½ teaspoon ground cinnamon
In a separate bowl, combine the buttermilk and vanilla extract and whisk together. Pour the buttermilk mixture over the flour mixture and stir together just until moistened.
1 cup buttermilk, 6 Tablespoons salted butter, 1 teaspoon vanilla extract
Drop spoonfuls of the dough on top of the berries, then sprinkle with the cinnamon sugar mixture. Bake for 35 to 40 minutes, then remove from oven and let cool for at least 1 hour before serving.