Peel and slice a ripe banana into 1-inch rounds, then freeze on a parchment-lined baking sheet for at least 2 hours or overnight.
Remove 1 large frozen banana from the freezer and let sit for 1-2 minutes to slightly thaw for easier blending.
Pour the milk into the blender first, then add the Greek yogurt, vanilla extract, frozen banana pieces, and honey or maple syrup if using.
Add the optional protein powder last, on top of everything, to prevent clumping around the blades.
Secure the lid, start blending on low speed for 10 seconds, then increase to high and blend for 30-45 seconds until completely smooth and lump-free.
If the smoothie is too thick, add 1-2 more tablespoons of milk and blend again. Scrape down sides with a silicone spatula if needed.
Pour a small amount to taste and adjust sweetness with extra honey or vanilla if desired, blending for 5 more seconds after each addition.
Pour the smoothie into a tall glass or mason jar and serve immediately with optional toppings like cinnamon, banana slices, honey, or granola.