Drop the frozen banana chunks into a clean shaker cup or wide-mouth jar. If the banana is frozen in a solid block, give the container a firm tap on the table to break it apart.
Add the powdered milk, salt, and vanilla extract (if using). Pour in the cold water.
Screw the lid on tight. Hold the container with both hands—one on the lid, one on the base—and shake vigorously for 20 to 30 seconds. You'll hear the chunks break down and the mixture go quiet as it thickens.
Open the lid and check the texture. If it's too thick, add another tablespoon of water and shake again. If it's too thin, add a teaspoon of extra powdered milk and shake briefly.
Drink immediately, while it's still cold and thick. The smoothie will separate if left to sit.