Prep the peaches: Peel and pit fresh peaches and cut them into chunks, or measure out frozen peach slices directly from the bag. Fresh, ripe peaches should feel slightly soft when pressed.
Add ingredients to blender: Add the peaches to the blender first, followed by the Greek yogurt, milk, honey and vanilla extract. Layering the fruit first helps the blades pull everything down evenly.
Add ice if needed: If using fresh peaches, add the ice cubes now for a cold, thick texture. Skip this step if your peaches were already frozen.
Blend on low: Start blending on low speed for about 10 seconds to break down the larger pieces of fruit before increasing speed.
Blend until smooth: Switch to high speed and blend for 30 to 45 seconds, until the mixture looks completely smooth with no visible chunks or peach skin bits.
Check the consistency: If the smoothie is too thick to pour easily, add milk one tablespoon at a time and blend briefly. If it's too thin, add a few more ice cubes and blend again.
Taste and adjust sweetness: Taste the smoothie and add a touch more honey if the peaches weren't fully ripe. Stir in the cinnamon here if using.
Pour and serve: Pour the smoothie into chilled glasses right away and serve immediately while it's cold and slightly frothy on top.