Cut avocado, remove pit, and scoop flesh into a blender. Peel banana and add to blender.
Pour milk into the blender first, then add Greek yogurt.
Add vanilla extract, optional sweetener, and a pinch of salt.
Blend on low speed for 10 seconds, then increase speed and blend until smooth and creamy.
Stop and scrape down sides if needed, then blend again briefly.
Add ice cubes if using and blend until cold and slightly thickened.
Taste and adjust sweetness if needed.
Pour into glasses and serve immediately.