Add liquids first. Pour the milk into the blender. This helps the blades catch and blend everything smoothly.
Layer the rest. Add yogurt, banana, oats, frozen berries, vanilla, cinnamon, and a small pinch of salt.
If using sweetener, add 1 teaspoon to start.
Blend on low, then high. Start on low speed for 10–15 seconds, then increase to high and blend until very smooth, about 45–60 seconds.
Adjust thickness. For a thicker smoothie, add a few ice cubes or a bit more frozen fruit. For a thinner sip, splash in more milk.
Taste and tweak. Add another teaspoon of honey or maple syrup if you want it sweeter, or a touch more cinnamon for coziness. Blend briefly to combine.
Serve right away. Pour into a chilled glass for the best texture and flavor.