Add the frozen mixed berries and sliced banana to a blender jar.
Spoon in the plain Greek yogurt and add the vanilla protein powder on top of the fruit.
Pour in the milk, then add the honey and chia seeds.
Add the ice cubes to the blender.
Blend on low speed to start breaking up the frozen fruit, then increase to high and blend until completely smooth and creamy, 45–60 seconds, stopping to scrape down the sides if needed.
Check the consistency; if too thick, blend in a little more milk, 1–2 tbsp at a time. If too thin, add a few more frozen berries and blend again.
Taste and adjust sweetness with a bit more honey if desired, then blend briefly to combine.
Pour the berry protein smoothie into two glasses and serve immediately while cold and thick.