Add the milk to the blender first, followed by the Greek yogurt and vanilla extract.
Add the frozen strawberries, blueberries, raspberries, and banana. If using honey, spinach, chia seeds, or protein powder, add them now. Layering the liquids first helps the blender create a smoother consistency.
Blend on high speed for 45 to 60 seconds until completely smooth and creamy. If the smoothie is too thick, add a splash of milk. If it's too thin, blend in a few more frozen berries or ice cubes.
Taste and adjust the sweetness with additional honey or maple syrup if desired.
Pour into chilled glasses and serve immediately.