Pour the milk into a blender, then add the protein powder, nut butter, frozen berries and spinach leaves.
Puree until smooth. My blender takes 54 seconds, but you may need to scrape the spinach leaves off the sides and give it a few more pulses to thorough combine. Check consistency and add ice cubes as needed.
Pour into a glass and serve immediately, or cover and store in the refrigerator for up to one day. Stir or shake to combine before serving.
Notes
You can substitute kale for the spinach, but it has a much stronger taste (and less iron). You may need to add a banana or some form of sweetener to blend the flavors better.