Freeze peeled, sliced bananas for at least 6 hours or overnight until solid.
Place frozen banana slices, frozen mixed berries, milk, and vanilla extract in a high-speed blender or food processor.
Blend on low, gradually increasing speed, stopping to scrape down the sides until mixture is smooth and creamy, about 3–5 minutes (add additional milk, 1 teaspoon at a time, if needed).
If desired, blend in maple syrup or honey until evenly combined.
Scoop immediately into bowls and serve for a soft-serve texture.
To store leftovers, transfer to an airtight container and freeze up to 1 week; let soften at room temperature for 10–15 minutes or pulse in the blender before serving.