Prep the slow cooker: Lightly grease the insert with nonstick spray or a thin layer of butter to prevent sticking.
Make the berry base: In a large bowl, combine the mixed berries, 1/2 cup granulated sugar, brown sugar, cornstarch, lemon juice, and 1 teaspoon vanilla. Toss gently until the berries are evenly coated. Pour into the slow cooker in an even layer.
Mix the dry topping: In a separate bowl, whisk together flour, 1/2 cup granulated sugar, baking powder, salt, and cinnamon (if using).
Add the wet ingredients: Stir in the milk, melted butter, and remaining 1 teaspoon vanilla until just combined.
The batter will be thick and spoonable, not runny.
Add the topping: Dollop the batter over the berries in 8–10 spoonfuls. It won’t fully cover the fruit; that’s fine. It will spread and puff as it cooks.
Cook low and slow: Cover and cook on Low for 2.5–3.5 hours, or until the topping is set and springs back lightly when touched.
The edges may brown slightly, and the berries should be bubbling.
Avoid peeking: Keep the lid closed for the first 2 hours to maintain heat. If you must check, do it quickly.
Rest before serving: Turn off the heat and let the cobbler sit for 10–15 minutes. This helps the juices thicken.
Serve: Spoon into bowls while warm.
Top with vanilla ice cream, whipped cream, or a spoonful of Greek yogurt.