Peel the ripe banana and break it into chunks. Place the banana chunks, frozen mixed berries, plant-based milk, chia seeds, nut butter, and optional sweetener (maple syrup or agave nectar) into a blender.
Blend on high speed until all the ingredients are well combined and you have a smooth and creamy consistency. If the mixture is too thick, you can add a little more plant-based milk or water to achieve your desired consistency.
If you prefer a colder smoothie, you can add a few ice cubes to the blender and blend again until the ice is crushed and incorporated.
Once blended to your liking, pour the smoothie into a glass or jar.
You can optionally garnish the smoothie with some additional toppings, such as a sprinkle of chia seeds, a few fresh berries, or a drizzle of nut butter.
Give the smoothie a quick stir before enjoying it. Sip and savor the refreshing flavors of this vegan smoothie!