Prepare the banana: If you haven't already, peel and slice a ripe banana, place the slices in a freezer bag, and freeze for at least 2 hours or overnight.
Add ingredients to blender: Place all ingredients in a high-speed blender in this order: almond milk first (prevents sticking), then Greek yogurt, peanut butter, protein powder, flaxseed, cinnamon, and frozen banana on top.
Blend until smooth: Start blending on low speed, then gradually increase to high and blend for 30-60 seconds until completely smooth and creamy.
Check consistency: If too thick to blend properly, add almond milk 1 tablespoon at a time until it reaches your desired consistency.
Taste and adjust: Taste the smoothie and add a teaspoon of honey or a few drops of vanilla extract if you want it sweeter.
Serve immediately: Pour into a bowl if eating with a spoon, or into a glass if drinking, and add any desired toppings.
Adjust for sipping: If you plan to sip this slowly over 30-60 minutes, make it thinner by adding extra almond milk so it doesn't become too thick as it sits.