Add the frozen berries, banana, Greek yogurt, milk, protein powder, chia seeds or flaxseed if using, honey or maple syrup, and vanilla extract to a high-powered blender.
Blend until smooth and creamy. Stop to scrape down the sides or use the blender tamper if needed.
For a thinner smoothie, add more milk a splash at a time. For a thicker smoothie, add more frozen berries, banana, or a few ice cubes.