Slice the ripe banana and add it to a blender jar.
Add the vanilla protein powder, plain Greek yogurt, and cold milk to the blender.
Add the ice cubes, vanilla extract, honey, and ground cinnamon to the blender.
Blend on low speed to start breaking up the ice and banana, then increase to high speed and blend until completely smooth and creamy, 45–60 seconds, stopping once to scrape down the sides if needed.
Taste the smoothie and adjust sweetness with a little more honey if desired, then blend for a few seconds to combine.
Pour the vanilla protein smoothie into two glasses and serve immediately while cold and thick.